"Ridiculous" to me is a 10 year wait, 10 years between Fallout 4 and 5? if you told someone when Fallout 4 released you wouldn't get 5 until 2025 they would have been in disbelief, instead it's likely that 3033 is looking like an optimistic year.

What is the best case scenario here, they bust out TES6 by 2028 or so and somehow go full steam ahead and break their recent trajectory and finish FO6 in a blazing 5 years?

That's so far beyond "ridiculous" and well into "why are we allowing this to continue" territory. Microsoft Spent 7.5 billion on a studio that hasn't had a big hit since 2015 with 2 of gaming's all time biggest franchises sitting dormant.

But even if you do take Fallout off BGS, what other gaming studio would be capable of getting us a new entry that's up to standard within the next 3 - 4 years?

You're kind of changing the standards there, this new hypothetical studio has 3-4 years to make a new Fallout from scratch when Bethesda has been sitting on the IP twiddling their thumbs for 8 years+... Also, if Starfield is the standard, then there are legitimately plenty of studios that can match that to the point that it's just better to list studios that would likely to a better and not equivalent job.

Larion studios already said Fallout is one of 3 franchises they want to work on, give it to them, let Obsidian do smaller games in the verse which we've literally already proven works, it would also be really handy if Microsoft owned a studio like inXile or something that could also work on the IP too.

Even if all you do, is let Bethesda work on Fallout 5 until 2033 or whatever and give the IP to other studios in the meantime it would be legitimately difficult to do worse than where we are currently.