But there was a deadline for clubs to announce their interest in the new tournament. If the Anglo-French are bluffing about the deadline, then perhaps the Welsh could have passed the deadline and still negotiated further. Or perhaps the Anglo-French aren't bluffing and the Welsh would have lost hundreds of thousands by failing to join up. If I were Welsh, I would be beyond infuriated had the Welsh not joined before the deadline. The risk they'd have ran by failing to join the new tournament would register somewhere between losing a lot of good players, and administration/collapse. The funding shortage would have been so severe, it would have been a striking blow to the Welsh in both short and long-term.
Truth is, what could they have possibly gained by holding out? The English and French clubs could afford to wait it out for years in order for the others' involvement. The vast majority of the top clubs in England and France are either running a profit, or are backed by wealthy owners. Spending a few years waiting for the others to join would have been an easy proposition for the vast majority. The Pro12 clubs trying to make the English/French nervous by holding out would have been like a human having a hold-your-breath contest with a whale. You're going to hurt yourself a lot faster than you will hurt the whale.
And let's say both parties did hold out. If the current offer extended to the Pro12 teams was an equal share in the new tournament... let's say a year passes. The Pro12 teams are now in the midst of worrying about a funding shortage through not having a unified European tournament. They come back and start negotiating again. What stops the English/French at this point saying, "Well, we were going to offer you an equal share. But that was a year ago. Now we'll offer you less than that. You can either agree to it, or maybe you'd rather another year of funding shortage." What exactly can the Pro12 teams do? Time isn't their ally. Without unified Europe, the French/English may stagnate, but the others will downright suffer, and this increasingly weakens their bargaining position.