I'm not as familiar with European journals, but if this happened in the US (more or less forced open access) it'd possibly mean that every time we want to publish it would cost researchers ~$3000.
Hopefully this (and similar) legislation comes along with a statement regarding who's supposed to cover that cost. Because some of us avoid that price tag by publishing in non-open access journals.
I haven't read the specifics of this, but typically these kind of mandates come from funding bodies to cover the work they fund. So they're expecting that publication fees will be a budget item in grant applications.
Hopefully this (and similar) legislation comes along with a statement regarding who's supposed to cover that cost. Because some of us avoid that price tag by publishing in non-open access journals.