"I" for instruments ... but where is the "Select ALL" / Select None" toggle for visibility?

Hiding all staves except the one you are working would kind of sort of do a similar thing, so it's a reasonable thing to think about trying, especially if you didn't already know about the parts facility. And even if you did, if you were looking for an end-run around the fact that the parts facility is really optimized for generating all parts at once and being done, not opening and closing parts on the fly.

But it's not the same in some important ways. Most significantly. you are still working on the score, not the part. So any manual adjustments you make, any system or page breaks you add, any other formatting you do - all of that happens in the context of the score, not that part. Almost certainly not what you'd want. So you'd have to discipline yourself to only make changes to the actual notes and rests, not to the formatting, when doing this.

The actual intended purpose of the dialog has nothing to do with emulating parts, BTW. It's more about allowing you to have staves that are more or less permanently invisible, like if you need a staff for playback effect only (to write out some complex ornament, provide sample interpretation of rhythm sections parts in a jazz arrangement, etc.

But for MuseScore 4, the instruments panel takes on new roles and would be more suited for this additional use case as well. Most notably, it can be left open in a side panel, and the visibility controls are more prominent. But, still no buttons to quickly make all or none visible. Those would be good additions, I think.