I'm kind of focusing on Godot 3.x right now. Though 4.0 is fun, I don't think I can release a commercial game with alpha/beta software. I'm going to stick with 3.x for now and start my game, which may take 6 months to a year, and if Godot 4.0 is stable by that time I will port.
Also, OpenGL (especially GLES2) works on some seriously low-end hardware, like Intel laptops. Even my Decay demo was able to run on Intel (albeit at 30fps, but it worked). Moving to Vulkan means people will need newer computers and the better graphics also mean performance will be slower (unless you cut down on all the effects, in which case it won't look better than 3.x).
But we'll see what happens. Vulkan, in theory, should provide faster performance than GL if coded correctly. And mostly every GPU made in like the last 6 or 7 years should support some spec of Vulkan. It is just a matter of if Godot specifically is taking advantage of that, and so far I would say no.