There is a way around this, its playing the stock(some) maps, or custom maps by me and dirty. They are symmetrical, but we can choose where team locations are and then you can use capital victory. So you force them to lose once you kill their capital.

I explained myself poorly, so I edited and reworded the original post.  I was talking about when only one loser is left, after the rest of his team politely quit.

But I understand what you are saying regarding 'taking one for the team'.  That's different, but I still think if he can't get a colonizer out, he should still be dealt a defeat.  I believe after that, he still has the option to 'Keep Playing', and might still be able to help his remaining team win.  But I still think he should lose (if no colonizer).

Yeah, maybe they should add another victory condition/s (rather than a default condition). 

If #1 & 2 were fullfilled, what to call the Victory condition?   Maybe:  Imminent Victory?  No Hope?   Extinction Victory? 

And... I think this all may be related to not being able to achieve 'allied victory' in unlocked online games. 

i fight to the last man - more fun playing a game you are likley to lose - i quit AI & online games when im winning - but your safe .i dont play online that often

the last few games i have played against 8 ai - ffa AI have been fun i build 2 colony caps and send them out into space exploring the map - i build nothing in my home system - eventually pirates arrive and destroy while my homeworlds mines but i keep exploring - eventually you can sometimes find a ice or a astroid hidden beind a pirate base on the outer edge of the solar system - take it make it your new home - try to do this before the AI kills your own homeworld.

Then up the game speed and start building your empire - eventually you have to expand because you can only build so many ships if you only hold 1 world - but dont take the pirate base or kill any of the pirate ships just jump past to the next system - your homeworld is secure because the AI has to fight past pirates first meaning you dont have to spend your limited resources on defenses except for a stabase and enforced loyalty.

although i do remember having a single stealth tank left over after my base was destroyed in cnc (original) and the guy quit the game after he ran out of money because i destroyed ever 10 of his harvesters after about 2 hours of gameplay

Seems to me that this is more a polite social norm rather than a technical problem.  If there are players who do this, and you don't like it, then ou should black list them and not play against them.  It's a viable strategy....just not exactly a friendly one.  Better, though, would be to play with one of the existing victory conditions, like diplomacy, or the one where you have to destroy thier homeworld.  At any rate, ot doesn't seem like an issue that the DEVs should try and fix with additional code. 

My two cents....