I'm not sure it matters much to Sky if a small minority of customers chose to use routers that don't support MAP-T.
For the user, they'd either end up allocated a dual-stack IPv4/v6 configuration as today, and that's just one of the IPv4 addresses Sky can't share with other users, or they end up with IPv6 only depending on Sky's preference. Of course, Sky could just eventually switch off support for all non-MAP-T routers.
From a Sky perspective, if they support users with their own routers without MAP-T, keeping in mind they've got control of the majority of users' routers (either by a natural churn/upgrade cycle giving them MAP-T routers, or by introducing software support for MAP-T in the older Sky hubs), then they've increased their IPv4 'capacity' by a little less than 8x (with a 8:1 default MAP-T sharing ratio). The odd user who wants their own router isn't going to negatively impact the significant uplift in their IPv4 capacity that MAP-T brings Sky.