The benefit with an ONT (or even DOCSIS dumb modem) managed by the ISP is that they can do fleet upgrades much quicker as they don't have to keep all old protocols running. For instance the GPON -> XGSPON upgrade that some ISPs are running right now (or DOCSIS 3 upgrade) really only works well if you can turn off the old protocol which requires swapping out all ONTs/DOCSIS modems.
If customers bring their own stuff then you're stuck with these things for much longer.
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.
That's either a horde that understands the issue, or is an even smaller subset that is going to be a pita anyway.