There is no way to do both single and multi in Sims without sacrificing more of what we've already lost since Sims 2.
All you really need to do is look at how multiplayer games are played, what's in them, how they are hosted, and compare all that to Sims. Look at the other Sims games, then look at Sims 4 (which others have already pointed out was supposed to be an online game). Look at SimCity.
Multiplayer is fine for people who want it, and if they go that way there will be happy players and new players out there to embrace it. But the core Sim players will jump ship because it won't be the same.
In the grand scheme of things, does that matter? It depends on the size of the fanbase that wants multiplayer. If it's big enough, they'll move on and be fine. If it's not, Sims will die. Since I don't want Sims to die, I'd rather they not chance it and go back to their roots.
