Courts tend to follow the law, even when it might piss of business or/and government. Whether they'll get involved in commercial decisions will depend on judges' view of their authority. So that shouldn't get in the way.
The law that is being broken by charging for a carry-on bag is the Spanish law that I referenced earlier in the thread, which has been upheld twice (in 2019 and again in 2022), and in both cases the Spanish commercial law judges relied on European law. So the jurisprudence seems pretty clear.

To the practical, operational question of "what to do when the bins are full", obviously the same thing the airlines do now - gate-check. And maybe, once the law finally forces airlines to stop being so abusive, and following the law causes the airlines some operational challenge, the airlines will modify the service a bit to better adapt. Yes, each seat might cost €1 extra.
To your legal question about why carry-on luggage is legally required but checked luggage is not, it's because of the letter of the relevant law. (based on EC 1008/2008, and the Spanish national law relating to air passenger carriage). Similarly, as to quantity/weight - it's all in the relevant laws and in the existing legal decisions.
You can read the legal opinion here: https://www.studocu.com/pl/document/...aleta/83826960