Do you remember when in the SimCity your friends could come visit your world and place as many parks around as they wanted, effectively bankrupting your city while you were offline?
As a co-op game we'd have similar difficulties, even if we could be in control of our environment. As a multiplayer, we'd be opened up to potentially playing with minors and being subject to the environment we enter. Either way you're taking away the control a lot of simmers love about the game.
Not to mention, we all know how EAxis has been struggling to meet community expectations - do we really think having them focus on yet another element "adding" to gameplay will lead to game improvement? I predict it will be a never-ending nightmare with challenges that multiply as quickly as they're solved. After all the efforts they had to go through to make amendments to angry fans following the horrific launch, the blackout, and ensuing SimCity disaster, it'd be surprising to see them willingly subject themselves to another such injury - especially since they have much much farther to fall if this doesn't work. If we recall, those amendments did little to placate the angry community. I know 10 years is a long time, but I don't think it'll have been long enough to wash the foul Simcity taste out of my mouth to be wanting to try online Sims5.
(My intention was not to come across harsh, but to be bluntly honest)
