I see a chicken egg problem: Why would developers/companies abandon non-QUIC versions of their software if there is a significant chance users can't use it?
(If they even implement it under those conditions at all.)
Then, in a few years, when you notice that most of the traffic is done over QUIC, you still won't do anything as it has no significant cost anyway and helps the few % of people who haven't upgraded their network. And customers who can use it won't notice much as your website still works anyway, but your latency metrics should be better for those users.