WUT
Seriously, that is the biggest problem if you don't leave the game if you only have a few planets left, and a couple of ships, only the very best players can do something about it (forgive me as I am unfamiliar with the current rankings to know who are the best nowadays). Not everyone finds it fun to roam the map hopelessly with his small task force and trying to build up one or two planets somewhere if his team is suffering at all fronts (fun level is lower than starting a new game).
If it is 4V5 that can still be a win for the four, but it is an extremely rare thing to win against more players like 2v5 (and against as skilled players as your teammates are), but if there are noobs, of course the situation can be different.
I think this would be a better advice: ONLY LEAVE IF ((THERE IS NO CHANCE OF WINNING AND)) YOUR TEAM ALREADY AGREED ABOUT IT. this is the most correct situation here, not the "never quitting" that you suggest...
Oh, and do you want to force everyone to suffer for several long minutes until their forces are completely destroyed and are forced to leave (defeated)? It is not much fun to see your situation deteriorating without any chance to survive, unless your team is willing and can provide you a new "homeworld"?
I think quitting is acceptable if the player is somewhat nervous about the annihilation of his main fleet or walking into a crippling trap, or surrender if he sees no chance for his team to win, dragging the game for hours even if it's clear the other team already won.. In this situation a ban is the solution.. (ofc if that player does this many times).
AI powerful? lol am I missing something? Back in the days AI was crap, and clearly not a good replacement of a really skilled player... has it changed? Do you get a cruel AI now if someone quits and not a goodfornothing one?
Yes it can defeat even skilled players, BUT only if the player is attacked by a strong opponent AND the AI at the same time at different sides.