first of all right clicking the slider would delete it so you would need a different keybinding for it.
Second, how is this any faster than making a timing point? you can just press "o" (or whatever your keybinding for that is) and it makes a timing point there and you can basically immediately change the SV along with other aspects of the beatmap. Adding a separate keybinding would literally do the same thing, because you'd still have to type in the slider velocity.
this only saves like 4 seconds if you want only one specific slider to be slower or faster than the rest of the sliders. Granted this is a situation that has happens reasonably frequently, but it's not something overwhelming time-saving, such as the create polygon tool or the change slider into stream tool. It just seems like an extra keybinding that isn't really necessary.
Variable tick rate is a completely separate issue and most likely will never be supported, so that is a problem in it of itself.
Though to be perfectly honest, i don't have a problem if this gets implemented, but personally, it's one of those things that can just wait.