Removing unhappiness in specific situations would be unmanageable for the AI, because it can't plan ahead. It might capture a few cities, then when resistance ends, the AI suddenly drops to a happiness level it cannot recover from.
Question is, does the AI really factor in happiness to when and where to launch attacks right now? If it doesn't, which I suspect, then the above is moot. In any case it would be exceedingly rare for the AI to be able to storm itself to this situation, taking several cities in ~20 turns. It would be brilliant if only AI assaults were well enough co-ordinated for this to be a problem. And if it sometimes rarely happens, and there's revolt within a AI civ, it would actually be kinda fun and different.
Ahriman said:
The design is supposed to *deliberately* be one where conquest is slowed, and where you can't beat the enemy army and just rapidly capture or burn down their empire.
Yes, it's deliberate. And I totally agree with you when capture is concerned. And in civ4 it was quick capturing (well, at least after the middle ages it was, when growing upkeep costs wouldn't overwhelm income from a large empire).
But it should be, at least in my opinion, that you *can* burn down the enemy if you can beat their army. Cities also fight back in Civ5, hard in earlier periods, unlike Civ4, so there is slow down. You can remove a neighbor this way, but you didn't get any stronger compared to other civs. In fact you probably invested a lot in those units and lost some, too, so are now behind. Yes, you now potentially have free land, but settling that needs happiness just like capturing it would. So why deny warmongers that?
Needing happiness to burn cities is deliberate to drag on conquest, but is a artificial and contrieved way to do it, linking it to the governance efficiency problems of a large empire. Far better way would be to link it to the military force. Instead of demanding happiness, cities being razed could, for example, create partisans that pop up near the city. Much more realistic to need an army rearguard left behind during razing, to defend against partisans, than to need colosseums and theaters.