This is such a common thing and tolerated you have to wonder whether it's actually immoral. I've met many people on my travels who went to Europe on tourist visas, got work and then got to stay legally later. No one was deported.

All of these were people in low-paying services industries, jobs Europeans don't usually want (waiters, cleaners, etc).

The only ones that had issues with immigration were my qualified worker friends who got a work visa and then the company had layoffs while they were there, losing their sponsorship. People with masters degrees who had to scramble to find new work in 30 days or face deportation.

It's hard not to think that's intentional.

I have a nuanced opinion because it's a rather complex subject but it's just a weird thing to have seen happen. As a tourist I had to prove up and down I wasn't going to stay there only to see no one else cares outside the airports. There's obvious wage suppression going on with these policies but these waiters and cleaners also had college degrees from good institutions, probably more qualified than some citizens.

What do you mean by valid legal grounds? For many countries all you need is to get a local job paying above a threshold, that’s enough to get a work permit.

Now I'm curious what countries we're talking about and what's the process of "legalisation"

How does this happen? Is there a law which just gives you a citizenship if you stayed for N years?