That’s a totally fair concern, especially as a long-time player. I agree that the single-player experience should always be the priority, and I don’t think anyone wants that compromised.
My suggestion isn’t a full multiplayer overhaul, but rather very small, optional, and contained features that could be tested incrementally if they can be done without pulling resources from core single-player development. I also agree that a fully realized multiplayer Sims would probably make more sense as a separate standalone title.
On a personal note, I hope some form of optional multiplayer could exist one day mainly so my cousin and I could enjoy our favorite game together when we’re apart.