Historically, this is what happened. Large empires were rarely brought down by external enemies, they were usually killed by internal issues. However, EU4 completely ignores the internal issues that large empires will have, you could own the entire world and so long as you have everything cored and converted to your religion, you will have no worries of revolts.
Given, I doubt anyone is asking for warscore limitations to be completely eliminated, but rather instead of being only able to take, say, 15 Basetax, you could take 30 or 45 basetax. It still wont be enough to vassalize France, but it still allows some pretty brisk conquest.