The ETIAS will last for three years I believe, so it’s just a check every now and again to make sure it’s still valid and a reminder in your phone a couple of months before it expires. As a definite non fan of Brexit, a bit of extra paperwork every 3 years causes me no issue
Think, at least in the first 5-6 years, there will be many (millions as this affects everyone from outside Schengen??) arriving at the gate to find they can't get through as either one or the other has expired, thus (a) holding everyone else up in the queue and (b) the authorities then having to deal with them.
Currently, of course, at ibiza immigration they can retain anyone they like, but that would be a handful over a week, maybe year, but what if they have to detain dozens, daily, who won't have a return ticket for days/weeks? What to do with them??
I suppose (a) they can do an emergency ETIAS there and then but that's taking officers away from control (longer queues?) or (b) insist airlines check the ETIAS individually before allowed onto the plane.
But (b) leaving from Ireland the airline would have to check whether they are Irish passports (allowed on) or UK (allowed on but only if ETIAS-compliant); again a bit of a bureaucratic time-waster, and even worse in the UK airlines would check if they have an EU passport (allowed on...) or a UK one with ETIAS.
And I'm not sure whether the ETIAS will be a paper document, or a stamp in your passport, or (more likely) simply an electronic tag (when you link to your passport, on swiping the officer can see it's compliant) and airlines don't have the facility to do that at the gate before you board.
After you...politicians. you made this, you sort it.