Does it make sense, in general, to prevent the hiring process from becoming more selective in the future? If so, why? But if not then a rule like that wouldn't make sense.

I don't think it makes sense to prevent it becoming more selective, but I do think it makes sense to avoid passing over candidates who can do the job. And your best (only?) data on who can actually do the job is who is currently doing the job.

If you really need everyone in Role X to have a PhD in Psychoergonomics, then what's up with Jane over there and her MD?

If you're hiring because you need someone? Yes?

It makes sense to sort candidates with the more niche requirement to the top of the pile, but to require it? When you need to fill this role?

There's also hiring because "wow, this candidate is great, we should find a place to fit them", and there it makes sense to become more selective going forwards... but when a company is saying "we need people and can't find them", that doesn't seem like the time to be more selective.

For some reason I fancy buying myself a new watch.

Anecdote: Amazon has a hiring bar that states that new hires have to be better than half of the current population in the role. Whether or not it’s adhered to, there’s a reasonable motive for doing so.