The major bit will be when and if toolkits like GTK, Qt, SDL, etc. drop X11 support and applications update to X11-less versions. AFAIK there are no plans for that and is still years away (there was some talk about GTK 5, but nothing firm and no one even started work on GTK 5).
I also think Wayland will eventually replace all of X11, in the same way as IPv6 will eventually replace all of IPv4.