This discussion is going to keep going on until we get a firm answer, but there's a couple of things I want to throw in the mix.
The first is, "they wouldn't miss out on this profit!" and "They have to make that money back somehow!" are both fundamentally wrong about why big businesses make these purchases. Bethesda isn't a cost, like you'd buy a kitchen appliance. It's a live, active business, which will bring in its own profits and have its own outgoings, and its own valuation (as well as adding to Microsoft and Xbox's own valuations).
Add to the fact that a Microsoft investment - especially right now - is about beating interest rates on its massive bank balance, and Bethesda doesn't need to make very much for the purchase to make sense.
If the company never released another game, but managed to make more than about £180m a year from its back catalogue, Microsoft and its stockholders would probably be very happy with their investment. That's not going to happen, obviously. We already know there are unannounced or pre-production projects on the go.
Bethesda doesn't have to "work" for Microsoft in order to be a decent buy. It's basically a decent buy by default. That's the benefit of having $140b in cash lying around.
Secondly, a few other people have mentioned contracts, and that's absolutely true. I can see this being the major reason we end up with Bethesda games on PlayStation.
We've already seen Bethesda are working on Xbox exclusive releases. As far as I know, the Wolfenstein, Prey and Dishonored collections still don't have a PS5 rating, but are coming to Xbox Series X. In theory, they're thrown together collections with minor updates that'll be available on Gamepass anyway, and not necessarily the kind of five, six or seven year project that Bethesda usually undertakes, so it may be that their exclusivity is misleading.
But it might not.
Starfield has been on its way for a long time. We know Sony wanted some sort of exclusivity.
My point is, whatever happens, we're not going to be at the norm when it comes to Bethesda for probably the bulk of this generation. There's going to be a transition period, and regardless of what happens with Starfield (or Fallout, or The Elder Scrolls, or Doom...) everything might change in seven years. Contracts will have been signed. Ideas will have been fleshed out. Microsoft might like the idea of releasing on PlayStation today, but hate it tomorrow.
This will be a fluid few years for everybody involved, let alone the fans. I reckon everything bar those two PS5 games will be exclusive, but there are so many reasons why that might be wrong that we'll never even know.