I don't understand the proposal, has it been shared publicly yet?
I am curious whether the clubs are proposing it as an additional competition to play in vs as a replacement of their participation in local leagues of the CL. Both would accelerate the concentration of wealth in the big clubs and would absolutely eat a lot of the market share of the leagues. However if its in place of participation in the national/local leagues then this is an unmitigated disaster for the UEFA competitions' market share.
Either way UEFA and FIFA have no choice but to oppose it. They currently hold a monopoly on everything Football they cannot let their biggest assets do something private on their own because they know fans will 100% tune in. This league will be a footballing spectacle I would certainly watch it!
It would be different if it's a new additional competition organized by Uefa. I wouldn't be surprised if the clubs and Uefa both double down initially for a year or so causing financial damage to both sides before then returning to the negotiating table and compromising on some form of UEFA organized super League. I doubt that the status quo will be maintained without change.