Some basic assumptions that you have left out of this discussion/thread:
Gametype. What game type are you talking about? FFA, 1v1, small teams, large lobpot?
In FFA you should never resign. As long as you have a con, you can have a comeback. See B1136649 6 on SplinteredTropicsDryV2
In 1v1, most skilled players avoid dragging out the game out of sportsmanship, but I think a player should be allowed to try and hold out for as long possible if they want because it is good practice, especially for newbie players, and the occasional big comeback is worth it IMHO.
In small teams, it can be fun to try and hold out against the odds. People can go and start another small team game if they want.
The biggest issue is the large lobpot, where players are just sitting around waiting for the next game to start without moving to another host and starting a whole new game.
To be honest, a lot of the time people start !resign polls way too early as people might just look at their own lane and start a !resign vote just because they have lost most of their units and don't look around the map to assess the situation.
Because of this, big comebacks are very rare in the lobsterpot as players are likely to !resign way before that.
Sometimes, a team even resigns with a just slight disadvantage or sometimes even a technical total metal advantage.
What if people want to do a last stand? Should that be forbidden in the lobpot?
Personally I think the worst type of players who wants a last stand, are people who just make defenses around their base and disregards the rest of the map because their play does not constitute good teamplay.
It makes sense to !resign before they get their last stand to avoid fostering such behavior.
If you know somebody on your team wants to make a last stand while you rather !resign and start a new large lobpot game, you can just keep playing or give away your army and cons to somebody, and then vote for !resign whenever it comes up again.