I think Musty very well could--and maybe should--be with the Sharks next year. The production this year is obviously really important, but there's also his size and strength. He might be young next season with his 19th birthday this summer, but not all 19-year-olds are created equally. With that in mind, it might be better for him to be with the Sharks next season than for Will Smith to be in the NHL, since Smith could struggle more physically.
And I can't help but think that keeping Musty all the way through training camp and seeing how he'd do in preseason games this season was a tease for this coming season. Anyway, he wouldn't have to play all 82 games; he could be used like Leo Carlsson, sitting out to rest/watch occasionally, and they could let him go play in the WJC on top of that. I think if he gets his nine game trial to start the year he's going to make it clear that this is the best path for his development.