It doesnt even need to come to that to be come an issue for them. This is a PR war at this point, and the mere threat of banned from domestic leagues (NTs is just grand standing IMO, but domestic leagues can be a real threat) becoming remotely close to official, can sway the resolve of some of these clubs. I'm pretty sure the Spanish and Italian clubs are all in (tho if truly banned from r Serie A, I would find it hard to see any of them being ok with pursuing this).
But the English clubs? They are the most image concious of all, if the heavy backlash and threats at home of expulsion increases in pressure levels, I would see them as the first to buckle and back out. And even with El classico duo still there, this shit would never ever work without the viewership power of the English clubs.
And yeah you can have 12 popular teams in an artificial super league together, but if the backlash is so heavy and anti the venture, and they are shunned from other venues, it will hurt their image and bottomline enough for them to seriously consider if this was really a sound venture. I wonder if the focus grouped this, and what their responses was lol.