In short, Wayland is a protocol for graphics.
It's somewhat similar to X, as its main purpose is the same, however the archivecture is very different, and Wayland is much simpler/barebone.
If X is going to die or not — only time will tell. For now it can be considered another competing standard.
Best: depends on needs and usecases. Obviously, the majority will name the distro they use here, because if they would think another distro is better they would switch.
Worst: either Pop_OS (the most pointless, with the killer feature of "nvidia blob out of the box", saving literally one command after installation), or Fedora (RedHat + too unstable + and too much RedHat new experimental shit being tested). Sorry for those who like those distros, but that's my opinion.
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.