Various places in Europe already have what amounts to at-will employment. There are exemptions for companies under a certain number of employees (e.g. 25 in Italy). There's a wide use of fixed-term contracts (6/12 months). Many work through agencies, which means they can be "fired" with a few weeks' notice.

Not really. Fixed-term contracts can not be used indefinitely. A worker must be permanently hired after the first extension. Agencies can not be used indefinitely, and also, the agency is required to support the employee after the client lets them go. So the company just pays to shift that responsibility but the responsibility towards the employee is there. A company is also not allowed to make an employee 'self-employed' by making them start their own company. They must always have multiple clients, if they have just one the government will consider it permanent employment with all strings attached and will apply all relevant restrictions and taxation retroactively.

I'm just talking about holland here but all over europe the conditions are similar.

All the exceptions you mention were just sly ways the companies have tried to circumvent their responsibilities and the law has caught up with regulations to make those impossible or at least impractical.

And there are some exceptions yes. But those are mostly for in-between gig jobs. Not for stuff people make a career out of.

Of course there are also exceptions with easy firing for things like gross negligence. Though the employee always has the ability to countersue.

Depends a lot on what country, but I think you'll find that the ratio of full-time employee vs contractor/at-will employee in most European countries will look very different from how that ratio looks like in the US (or other similar countries).