Whether or not Sims 5 is made will depend on how well the newly released Sims 4 does, a Maxis producer has said.
Speaking in a Q&A thread on the official forum, he noted that a potential Sims 5 would only be made if the 4th game did well.
If Sims 4 isn’t successful, there won’t be a Sims 5.
He also stated that currently the team was not even considering a fifth game, instead working to support the Sims 4.
We’re not working on Sims 5. We’re not thinking about Sims 5.