Project Rene seems to be very early in development and they seem to be split into different teams when making it, at least that's the impression I got from the recent sims summit because they all seemed to be working from different places and I don't think that will be enough to make a full game... but maybe they will pull it off?
Depending on what they are aiming for I think everyone at Maxis needs to shift to Rene when they have decided on everything that they need to make so they can go into full production. There were familiar simgurus working on rene in the sims summit video so I don't think it's unlikely that they will shift more and more people who have worked on the sims4 over to rene.
So they might still make content for the sims4 after rene is released, but I'm not sure that is something that Maxis would like to do because they have all shifted over? and maybe people would not want that either because the content will be made by different teams who might not make content that people want.
I think simguruAzure said that they had many ideas yet on what they wanted to make for the sims4 a couple of years back and what they said had come true, they talked about generations for example, but she was shown working on Rene in the recent sims summit as a producer. So I think it will be very much up to Maxis if they feel that they can handle making content for both games.
