I doubt all ISPs are going to follow on that. If only one big player is left with unlimited bandwidth, it would collect all the customers.
Only all the heavy users, that is.
I can't blame the ISPs, after all charging by usage (or limiting "over-usage") just makes sense, and is fair to the rest of the users. I think similar tendencies are in other ISPs around the world as well, as the data transfer rates just keep getting higher and higher due to both more and more mobile data usage, people downloading xx gigabyte games over and over again, and watching Netflix etc. online.
Most probably this is something that will mostly be applied to cheaper net connections, more expensive connections will probably still provide uncapped data transfers. You just have to pay more for them.
It is also probable that many big online service providers (Netflix, Steam, I hope GOG too, etc.) will have mutual agreements with the ISP(s) so that if you are downloading stuff from those services, they are not counted towards the data transfer caps. Those services probably pay something for the ISPs for getting that privilege.
Anyway, vote with your wallet, if you can.