It sounds easy, but LN address support would mean they'd all need to add centralization to their "self custodial" Lightning. Bolt 12, which Phoenix offers, is better IMO. There is no need for a third party server.
Discussion
It's a trade off, for sure. But an LN address opens up a lot of functionality. I don't know enough about the backend but would it be possible to allow users to hook to independent address services to establish and use an LN address? That way changes to the core model of those wallets wouldn't necessary?