SimGuruLyndsay confirms The Sims 4 has no planned End-Date.

If they are taking time to continue updating/overhauling the base game in the years to come then I think it's a good sign. However, if they just keep releasing standalone expansions to a really dull base game (which is what I think is going to happen, to be honest), then I am so over Sims 4. Just my personal thoughts.

I would like to move on to The Sims 5, but I also want to make sure it's done right - if that means more time to learn from The Sims 4 development cycle then I can wait.

edited for clarity.