Yep. Nope. It happens. It’s the internet. Be glad they didn’t kick you off their table. That was possible twenty years ago on another site.
Please resign instead of letting the clock time you out

Yes, please just resign

I hate it, but it's a fact of life. It always seems funny that the other person is clearly just irritated at losing and they think they are getting back at you. But since they are also sitting there wasting their time also (when we both could be starting another game) it reminds of someone that grabs someone and pulls them over a cliff. Both end up falling.
I believe it is immature, and unsportsmanlike conduct.

Guys, flag these stallers as "bad sports" and REPORT them!
We all need to start doing this consistently. That's the only way to weed them out. I don't actually know if chess.com acts on the reports, but I do hope so because why else would there even be a bullet point specifically for this issue in the report form?
Accruing enough "bad sport" flags supposedly kind of shadowbans you, so that you only play other people with many "bad sport" flags - which is a very efficient way to deal with these bad mannered players, at least in theory.

It's only happened to me in Bullet and Blitz which I don't really mind. It's only a couple minutes for me at max, but I understand how you guys are feeling in the 15|10 games for this to happen to them. On the upside, if you're a Premium Member, you can just start another game instead of wasting your time with them (you can play multiple games). If you're not one, just jump on another website or just use it as a break period. As annoying as it gets, you can't let it get to you by screaming at them in the chat.

It's clear that some persons are immature and are not ready to learn more even if they lose.
Network connection too can cause such to happen, I've played online and I was about to checkmate my opponent's king then I realized there was no longer network connection available to complete the game, I waited for the network to be back, unfortunately, I lose..... (Really painful).
So, we shouldn't blame them cos some while playing have different challenges

It takes a real loser to be gracious in defeat, but make no mistake; this is intent to cheat. There is a real possibility that you lose connection and then the game. Or what if you have to leave? When people do this, there is this miniscule percentage of games where they actually gain rating points for a game that in reality they lost; but technically, according to the web site, they are essentially gifted a win.
Chess.com is only concerned with cheating that involves the use of AI/software programs/engines or whatever you call it. If you're making your opponent play Stockfish while you take credit for it, they will close your account. Any other form of cheating, you or your opponent will get a pass or a slap on the wrist.
Letting the clock run, when the game is inevitably lost, is a form of cheating. It's not just bad sportsmanship - it's cheating.
Most people who do this, do it as a practice and general policy. It's not because they are angry and being egotistical, mad that they lost, etc.. They do it regularly. There have been quite a few times in a blitz game, I let the clock run, but I had only seconds left. I did it to give my hand/wrist a break or sometimes I was just stuck looking for a move. I received a warning and think of this. Would we get issued a warning if this wasn't something people are doing regularly?!
There is a multitude of bad behavior which goes unpunished. Unfortunately, this is viewed as poor sportsmanship and receives only a slap on the wrist.
It's cheating, people! Chess.com needs to punish this accordingly.
So if someone uses an engine, they are banned for life.
If it was possible, they would resort to draconian punishment for engine use. If only it was legal, chess.com would strap users up and pull appendages off, along with eye lids and toe nails. They might even pack salt in the open wound.

That being said, it is poor etiquette to simply let the clock run down. At minimum it deprives the player a lesson in loss. We all play, and we all loose. Surely, a key to understanding how to win is understanding how to lose.
It takes a real loser to be gracious in defeat, but make no mistake; this is intent to cheat. There is a real possibility that you lose connection and then the game. Or what if you have to leave? When people do this, there is this miniscule percentage of games where they actually gain rating points for a game that in reality they lost; but technically, according to the web site, they are essentially gifted a win.
How significant would the amount of points gained be in the long run?
While it is bad sportsmanship I'm not sure if it really is cheating.
If you play a 10 minute game then you have a right to use the full 10 minutes.
And how would you differentiate between genuinely running out of time, thinking a long time and abandoning the game?
Should there be a mandatory tempo? Like 1 move per minute in a 30 minute game?

@Can-eh-duh "I received an important call and simply had to walk away. Not that it was justified, but that it was unintentional."
That could happen to anyone, but this happens much too often in my games for that to be the primary reason.
Resigning is one of the great things about chess. If you know you have lost, you don't have to keep playing and it isn't rude at all, like it would be in most other games.

It takes a real loser to be gracious in defeat, but make no mistake; this is intent to cheat. There is a real possibility that you lose connection and then the game. Or what if you have to leave? When people do this, there is this miniscule percentage of games where they actually gain rating points for a game that in reality they lost; but technically, according to the web site, they are essentially gifted a win.
How significant would the amount of points gained be in the long run?
While it is bad sportsmanship I'm not sure if it really is cheating.
If you play a 10 minute game then you have a right to use the full 10 minutes.
And how would you differentiate between genuinely running out of time, thinking a long time and abandoning the game?
Should there be a mandatory tempo? Like 1 move per minute in a 30 minute game?
The points gained in the long run are absolutely insignificant. Just because someone's agenda is based on faulty reasoning, doesn't change the fact that it's still their agenda.
If you're threatening inevitable mate in 1, and your opponent wins the game because they waited until you were disconnected to move, that's cheating.
Why is it cheating? Because you won the game. Their rating went up, your rating went down. Do the ratings matter? Does it matter that it's on record that you lost? I'm not here to debate that. How is cheating defined?

What is worse, trying to cheat or succeeding at cheating?
Does it matter?
If someone fails at cheating, they are just as guilty as if they had succeeded. The action was the same, the result was different.
So letting the time run is not just bad sportsmanship, but it IS cheating. This isn't really a matter of opinion or perspective. It is what it is and it IS cheating.
Why isn't chess.com banning repeated offenders?
They ban people for trash talking.
Letting the clock run? They just restrict your account if you do it x number of times. As punishment, they pair you with other "bad sports," which could be whatever range of things. If people let their clock run x number of times, they should be banned; not restricted.
Here is a position from a 10 minute blitz game I played today. It is black to move.
My opponent (black) can see this is a lost position, so they do nothing and let the remaining 5 minutes 43 seconds run off the clock. This is very annoying and makes the game less fun for everyone. Please just resign and start a new game.
If we are all gracious in defeat, it will make the game better for everyone.
Does anyone else get tired of this happening?