A major motivation for flying is it's faster and that will still be the case, so they'll still have plenty of customers, right? So if an airline can fly a route for 20 euros but the minimum price is, say 50 euros, won't the airline just pocket an extra 30 euros?

Why are flights cheaper that trains, anyway? According to the article and the linked Greenpeace research, trains are 2-10 more expensive (and take longer) because of extra taxes that the airlines don't pay. So, instead of a minimum price, how about we address the root of the problem and either tax the airlines more or tax the trains less?

Maybe in addition to removing some exemptions, we add a pollution tax too (or maybe just raise the fuel tax)? Taxes have been used to motivate the market for a long time, so if we make it expensive enough to pollute, then it will motivate r&d to develop less polluting aircraft. In fact, hydrogen fueled aircraft are already being pursued: https://www.reuters.com/business/energy/hydrogen-aircraft-developers-are-long-haul-2023-02-09/

In my opinion, France's proposal is like using a sledgehammer to drive a nail.