For those deriding that it is only a ride, to many it's more than that, people form connections to memories and the joy something has brought to them. Disney more than any other park attracts people on nostalgia, certainly for Magic Kingdom.
This doesn't mean rides shouldn't be changed or updated, but on a technical level I can think of a dozen rides in more need of being refreshed or changed before SM did.
It does worry me that Disney could pander more and more to the Twitter outrage mobs, especially since most of those campaigning for these changes likely aren't even regular guests to the parks. To me it feels like they are pandering to people that aren't their customers, they should listen to their fans more and less to a vocal minority on Twitter, their money comes from one not the other.
As for myself I will be sad to see SM go, but will admit the concept art looks really nice, so if the end product is as good as the artwork maybe it won't be all bad, but I'm concerned this is the first domino to fall.