Maybe Bethesda will decide it wants to make Fallout 5 at some point but unless the franchise is in real trouble I don’t think they will. If that’s successful it can be continued and whatever developer they use can take stewardship of the franchise.
Now that they’re part of the Microsoft empire I think they’ll carry on with Starfield and The Elder Scrolls 6 and that sometime in the next five years we’ll hear of another studio, probably one of the others that Microsoft owns, working on a new spin-off in the vein of New Vegas. Especially after the disaster of Fallout 76, it’s clear that Bethesda are in no hurry to return to the Fallout universe and my prediction is that they’re simply not going to.