I’m not super familiar with desktop app development, but I don’t think it requires more work to keep supporting X.
It doesn’t depend that much on desktop application developers, but on GUI toolkit developers. It does need more work for GTK and Qt devs to support both. But the outcome will likely depend not that much on ammount of work as on “political” decisions. RedHat are now somewhat actively forcing Wayland in their distros. They also have their impact on GNOME, so it’s not impossible that due RedHat’s decision GNOME and then GTK (that is now developed mostly by GNOME developers, despite being GIMP Toolkit initially) will ditch X “just because”.
End user Application developers usually don’t deal much with Wayland or X — they just use toolkits (GTK or Qt for the majority), and toolkits do all the under the hoof work for them.