If you're referring to players who know they've lost and aren't actually playing the game and are letting their clock run down then I would agree with you (I don't know why they do this - maybe trying to bore you into abandoning the game so that their rating increases!), but if they're playing at a reasonable pace to try and win, and you think they should 'accept defeat' then I think that idea is completely absurd. The reason that professionals/GMs resign well before the end of the game is that a) they see positions further ahead and more quickly than the average player on a site like this, and b) they virtually always play the best move for the situation. Secondly you assume that neither player gets any benefit out of playing an endgame, but that is a skill that a lot of players claim that they need to improve. So instead of trying to wind your opponent up with sarcastic comments, why not use your advantageous position to practice your endgame and see how quickly you can checkmate your opponent?
You don't win a game of chess by being up in material, as your opponent can win on time, or can be materially down and still win via checkmate. I have done this myself, even after losing my queen when a lot of players would resign because they think it's impossible to win without their most powerful piece. If you resigned because you thought I'd lost and made a snarky comment then I'd be happy to take that as a win thank you very much.
[COMMENT DELETED]