Then we need empire collapsing mechanics, gov capacity offered chance for expansive ai to rely on vassals but alas
Yeah at this point we start bumping into the inherent ahistoricity of "historical" grand strategy games. Empires don't actually keep growing, progressing, and developing forever like they do in EU4 (or CK2, or Civ, or any other similar game) - they rise and then they collapse, because the idea of one person singlehandedly guiding national development through centuries is inherantly at odds with the way history goes.
This is very prominent in the devs' desperate attempts to keep Ming from snowballing spectacularly into a global superpower because that's what makes sense in the rules of EU4 but isn't remotely what actually happened.
I mean that's really a problem with most of the bigger nations in EU4.
The ottomans are just in the special position since they are already the strongest nation at games starting point and not hindered by game mechanics as Ming for example.
Spain's decline is also not really represented in the game. Yes they blob less than the Ottomans and take longer to grow, but they aren't nearly as weak by 1800 as they should be.
If we start to go that route (which I am not opposed to) we have to think about how this will affect player empires. Because if AIs get affected with a decline mechanic the players should be as well. As others pointed out the fact that we are basically immortal god emperors guiding nations for hundreds of years and leading them to unseen heights is fundamentally at odds with how the historical decline works.
In EU3 (or very early EU4 not sure about the details) most bigger Empires would eventually collapse (at least for certain patches). While this resulted in some much more historical games it also made it much easier for players to blob. Would the overall playerbase be fine with either having an expansion limit placed upon them or make the power gap between AIs an players even worse?