There is nothing preventing the EU empowering EASA to issue licences to former UK EASA licence holders either.
I'm sure operating EASA registered aircraft with EASA-licenced UK-based crews is financially the best option for Ryanair shareholders, in the same way employing pilots via bogus self-employment schemes or not recognising unions was until the regulators and courts caught up. There may be nothing preventing them doing this now, but who knows in the future.
Maybe Ryanair can use their considerable lobbying power to encourage the reciprocation of licence transfer.