Come to think of it, with all the new changes to the Sims 4 engine including coding updates and new python structure - as the Sims 4 engine is uniquely developed FOR the Sims I suspect they might be working on an upgraded engine for the Sims 5 rather then making a new engine or even attempting to transfer the game to frostbite (which would never work honestly and cost them a huge portion of players due to high system requirements which by large the community doesn't meet).
So if they are upgrading the Sims engine and smartsim technology they currently use for a new generation of the Sims (Sims 5) then from a development pov you'd spend that time right now deciding what new features and major changes you want to incorporate into the Sims 5 and then upgrade the engine to handle it. Which again would explain why they are giving the sims 4 an extended shelf life.
Heck with all the game companies announcing streaming gaming service, the Sims 5 could be an entirely different product like some weird streamed version of Sims Online.
