I agree that keeping the sims going forever and ever is a bad decision for the following reasons
- The game is limited by design choices made right at the start of the creation of the base game. For example the rigid worlds with zero freedom to change them. Sims 2 and 3 could do this but sims 4 never had this option. Some of these limitations can be remedied like toddlers but even then it then there are traces left. No toddlers spawn on community lots and no toddler playground equipment on any off lot playground. Other design choices that limit the game are things like swatches: even sims 2 had 2 color channels on objects so you could independently choose for example sheets and bed frames. Also the choice for emotions and how they override a sims personality are part of the game core. Lovestruck tried to fix this but it is a system attached to an existing system which makes it complex and doesn't feel very intuitive.
- Too much DLC makes the game unstable. The recent releases are full of bugs. And let's be honest, with so many features in DLC and so many possible configuration of DLC installed it is impossible to test all possibilities. That being said, quality testing should be done thoroughly for any release.
- Older technology : the sims is upgraded to DX11 when my two year old non-gaming laptop has DX12. When it comes to computers , technology moves very fast. How far can this go before it becomes a problem?
- Inspiration for releases : themes are revisited and redone in DLC. There's a lot of overlap. If every theme is going to be spread out over several DLC, it will become expensive to get the full experience. And let's be honest, the latest DLC is less beefy, less content for the same price.
- two farming EP (cottage living and horse ranch) ,
- two educational EP (University and high school),
- several vacation GP (outdoor retreat, jungle adventure and to some extent island living and snowy escape),
- family DLC (parenthood, growing together, kids room stuff)
I think a fresh start every now and then (it's been over a decade!) can be refreshing. You can leave behind the flaws in the game behind you and enjoy something new. Take the best of the previous iterations and build on that.