There are what seem to be major underlying architecture problems that limit any possible solutions - 6.1 brought back the familiar 'extreme server load' lockouts on populated servers blocking people from entering houses, even well after primetime in some cases. This is undoubtedly part of what lies behind the seemingly-arbitrary limits on apartments, too: the architecture simply cannot scale gracefully to meet the level of demand. In that light, "no instanced housing" is less a design choice and more a hardware limitation.

There's generally no easy fix to something like this - it's near-always months of engineering work by your most talented developers, laboring on what is most likely your most byzantine code that also happens to serve as the underpinnings for the entire game and cannot brook any significant level of failure.