If they are using vacation time to prolong lost games, you can report it to the staff. Otherwise, you agreed to the time control. It may be unethical, but I don't think it violates the rules.
To your idea: That wouldn't work well with people who are chess.com addicts. Some have more than 100 games at once! I've got around 50, and I wouldn't be thrilled if they implemented that idea.
Even though this is not an urgent matter, it's an idea i thing it might be taiken into account to address this issue.
I have noticed that some players while entering in a daily basis to make their moves in the online chess section, intentionally postergate the moves in those games their are about to lose.
I came to the idea that, if every time a player makes a move in any of his online games, a percentage of the time left in every game of that player is taken away he would be pushed to move in all his games and not only the ones he chooses.
All the games would be played quiker, players won`t lose interest in the game and tournaments length might get shorter.