Important enough I seek them out to spend time with their works.If the game doesn't happen, the other people, individually or as a group, will go find something else to do. There's a ton of other things for folks to do these days. Heck, they could also just find or play another game! This somewhat puts a hole in the idea of "no game" without the GM.
There won't be the particular GM's game if the GM isn't there. But there will be something else. There will be no unfilled void in anyone's life.
Go to the library (or, maybe your own bookshelf), and pull any one book off the shelf. The author is important because without them, that book would not exist? Now, look at all the other books in the library. There are tend of thousands of them! If the author/GM is only as important as the book/story they told... how important is it, really?
Does time continue to tick if something doesn't happen? Yes. Does that make my choice of what to do with my time less valuable? No. It makes it more valuable. Opportunity cost is a thing. If I seek out a game to play in and the game doesn't happen (especially with little notice whereby I am afforded no chance to reschedule), am I disappointed? I better be or I've done myself a disservice scheduling to be in the game.
Which participant is most responsible for that activity moving forward? The DM. Which participant is most responsible for me agreeing to play that particular game out of all the games available? The DM. Which participant has the greatest chance of driving me away from that game? The DM. Which participant is most important to that activiy? The DM.