Well this is about allowing customer supplied ONT, not supporting them. As in, you have to follow upgrade procedures announced X days in advance, etc.

In theory yes. In practice that might work that way if ~5% of your users are in that situation. If ~50% of your user base is running on a legacy protocol and you're running into Churn risks, the company is going to re-evaluate if they want to retire the old protocol.

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.

I mean you're right in general- but we're talking about a subset of customers that want to mess with their own fiber connection.

That's either a horde that understands the issue, or is an even smaller subset that is going to be a pita anyway.