I would imagine any such league would be deemed illegitimate by FIFA/UEFA so would probably mean associations booted them out.
I hope it goes ahead. I don't think they will be successful and I imagine most will be back with the begging bowl within a decade, where hopefully they are made to start again from the lowest tier and work upwards.
The Premier League is a unique product and generates supporters and excitement. It will continue to be thus if sides go. Fair enough the league may shrink somewhat, but we would be perfectly positioned to be the biggest fish in it.
If the PL had strength, it would call their bluff and re-raise those clearly involved in discussions. Make them illegal and say any club found to be negotiating such a league will be automatically demoted. Call the bluff. In all honesty if 5 teams are what they want (which would be about right) it probably only takes 2 to stay and it will then become a doomed concept in England and likely internationally.
What is stupid though, is to continue to allow these teams to negotiate while receiving the vast majority of the TV money's. Time to go on the offensive. If you want to go, go. If you are caught negotiating immediate demotion with say a 10 year ban on being allowed to re-enter the competition. See how many are brave enough to put their short sighted greed ahead of the common good then.