If your economy is too weak to support upkeep and you're being beaten by attrition, your problem is probably more in the early game than the late game. Keep in mind that the AI cheats for extra resources, but is otherwise an idiot. What's causing your stalemate isn't that upkeep is crippling your economy, but rather that the AI was too stupid to use its massive economic advantage earlier and kill you before you reached 2000 fleet command. You need to make your moves earlier on to ensure you aren't behind the AI when fleet sizes cap out. Taking more planets early on might be a good idea; I rarely, if ever, reach 2000 fleet command because the game is usually won before this point is reached. A great way to get ahead of the AI is in capital ship levels; I find the AI loves to get these guys killed, so you can have multiple level 10's late game when it's still running around with level 2's and 3's because they die so often.
The whole point of upkeep is to prevent stalemates. While AI idiocy (as usual) is notwithstanding, the idea is to allow stronger economies to crush weaker ones in a battle of attrition. If you can't keep up in fleet size, and can't compensate with some other strategic or tactical advantage, you will lose. The AI unfortunately is too stupid to apply its economic advantage, and can lead to the silly stalemates you describe, but in multiplayer upkeep is very important for breaking stalemates.
A large empire late game can easily be earning over 200 credits per second. Even with 75% upkeep, this is often more money than you can actually spend! Chances are your problem is that your empire is too small for how late in the game it is, and you waited far too long to get out and expand your territory.
