To be fair, Ronald Reagan opened the door to actor types running.... and Arnold Schwarzenegger, though he hasn't really done any acting in a while.
Reagan was at least governor for 8 years. He actually had a political career.
Ditto Arnie. Though (Demolition Man future-history aside) the rules didn't and don't allow him to become President.
I don't think Boris renouncing citizenship even disqualifies himself from the first condition required for the Presidency (the NBCness). And, even if it does, that can probably be re-reversed in the future. Especially during the fourteen years or so he'd need to plan ahead for to fulfill the necessary residency requirement.
What
might put the nail in the coffin, for us trying to play Fantasy Whitehouse League, is if he gets and accepts a seat in the House Of Lords. The unratified Titles Of Nobility amendment would, if made applicable and
not then overridden by Congress anyway, possibly exclude him so long as he materially benefits from the role.
Realistically, though, his (current) rejection of US citizenship
1 is no future bar. Not compared with just making an active/passive deliberate decision to just not move to the States for the best part of a decade and a half, presumably having lost all possibility of usefully serving within the UK government (or even in opposition, biding his time for another go). And he's not getting any younger. (Which at least means we don't have to factor in his disqualification from POTUS due to no longer fulfilling the minimum age requirement, however the other conditions turn out. Well, unless he uses the secret of rewinding time, known to just a select few. But obviously not being a Bullingdon Club secret, or else we'd never have been allowed to hear about some of the things they did.
)
1 More to do with his UK ambitions than any rejection of US ambitions.