As you're already aware, this is a terrible plan.
My parents retired to rural France in their 50s, over 30 years ago, so had residency and were in the health system pre-Brexit. Even so, the last 7-8 or years have been very hard, with Mum dying there 7 years ago and my dad (86 now) declining but not wanting to leave ever since.
Yes, the healthcare system can be good, but there are gaps in it. Things seem to be ignored which would be sorted here, because they want to keep elderly residents at home, so seem to turn a blind eye. No waiting for anything though.
And of course, rurally, everything is a drive, sometimes a pretty long drive, away. There are taxi-ambulances, if you can arrange one, but normal taxis seem non-existent. Relying on friends can be tricky. Your parents wouldn't even have a social circle to turn to. We'd noticed that friends and neighbours had faded into the background over the last few years, presumably terrified at the prospect of being responsible for him (I don't blame them, to be honest). I know people talked about us (his children), thinking we were neglecting him.
For his last 6 months there we took on a local lady as his PA. She speaks excellent English and has been a life-saver, dealing with bureaucracy, tax which would Dad wouldn't cope with any longer, taking him to appointments etc. It was hard to get him to accept he needed the help (he's never had anyone working for him to clean/garden etc), but he came to like and rely on her.
After some health issues, we finally managed to persuade him to return to the UK last spring. We'd originally looked at retirement flats with a warden, but it became clear that he wouldn't cope somewhere new independently so we found a lovely home. Several professionals have commented that he probably would have managed independently if he'd made the move some years before, but he'd left it too late.
Overall, my parents had 25+ years of very happy retirement, but they should have planned for being older and not pretended it wouldn't happen to them. The upshot has been a very hard and dispiriting time for my sister and me with endless trips back and forth and our own lives put on the back-burner, at the time our parents were enjoying their escape.
I don't envy your situation. If your parents have capacity, you can't stop them doing what they want. But perhaps you should talk to them and point out how hard it could be for you. Perhaps invent a good friend who's going through a similar situation with parents who've declined? If they insist on going, try to find a contact who can be paid to help them, who knows all the systems. Dad's PA has been worth her weight in gold and is still working for him as we try to sell his house.
Good luck.