Why? There is nothing preventing an ISP from saying that from date X, only protocols A, B and C are supported. If you want to use your own device, make sure it supports these protocols.
In other words, the requirement to allow customers to use their own devices does not mean that they can choose all available protocols. The allowed protocols can still be controlled by the ISPs.
It's like saying that Spotify could suddenly decide to retire support for Android 12 or something. They could, but how many customers are they going to lose and how much support burden is that going to generate?
The major difference is that the ISPs in this particular case do not need to offer any support apart from listing standardized protocols which are supported. If someone brings in their own device, it is on them to set it up and make sure it works with the currently supported protocols.
Also, this business model is nothing new. For example, mobile network operators have been using it for decades. Their base calling services might remain working even on the oldest phones. But when it comes to data services, they are gradually upgrading. Many of them are switching off their 3G networks to free up frequencies, for instance. Millions of people are affected. And yet, there is no drama around it.
One of the reasons might be that these phase-outs are announced and planned very long time in advance so the customers have the time to prepare. And they have a choice. Either upgrade their phones or live without fast-ish data or switch carriers if possible. Which is fair, in my opinion.
It seems to me that the ISPs could use a similar approach and be just fine.
A lot of overhead for ISP support in those cases in which a customer knows they can buy any router with any ONT, plugs it and forgets it without zero knowledge of what a protocol even is.
There _is_ a reason even legacy cable TV and ancient DOCSIS channels are still being available in many countries because actually retiring a lot of old modems has shown to be risky to the business.
That's either a horde that understands the issue, or is an even smaller subset that is going to be a pita anyway.