When will the Sims 4 become too taxing for average users pc? 29 packs currently...

I think and this might sound crazy since I'm talking about system performance, for the game to survive into the next decade they will need to not only up the texture sizes to 4k but improve the games underlying foundation/performance.

Otherwise the games textures will start to make the game look dated by 2021. If they plan to keep the Sims 4 as a service game - then they need to approach it as such and that includes improving the graphics. That doesn't mean an overhaul - just higher textures.