Why does it bother you that Disney decided to take character greetings to a whole new level of immersion? To me, that's Disney doing what it should be doing -- setting the bar for theme park standards.
Lots of people can't handle riding Space Mountain, but I doubt that many of them say "I can't believe Disney wasted money on this when they could have build an Omnimover that everyone could enjoy".
The mountain attractions aren't for everybody, I've ridden Space many times, but I do find it a bit too fast for my taste, and hence don't ride it every time. But for many kids, the mountains are a rite of passage, i.e. being tall enough to ride Splash or Space. The restriction is physical, i.e. the height restriction or the speed of the ride.
When looking at the rides at the other end of the spectrum, such as the Storybook Land Canal boat type rides, it is perfectly acceptable for adults to enjoy these attractions, even without kids in tow. As a single adult male, I can ride Snow White, Peter Pan, all of this stuff, without a cast member bating an eye lash.
Remember, Walt didn't want a roller coaster in his park, only after others pressured him was the Matterhorn built. Walt wanted 100% of the attractions to be 100% enjoyable by the whole family, letting roller coaster rides in his park was a concession.
I think the issue with the Belle's is that it is sort of new for Disney to build an attraction which would be awkward for adult guests without little kids to enjoy, even on Casey Jr. adults can enjoy the ride without raising eyebrows.
Remember, there are also a lot of families at Disney with teenagers and mom and dad who would feel uncomfortable in such an attraction, as well as plenty of folks without kids who would feel uncomfortable lining up for such an attraction. Yes, Walt had Tinker Bell walking around talking with guests, but nobody felt excluded, with this Belle M&G more guests are, in essence, "unwelcomed" than welcomed by the attraction, so it is a sore point for folks to see money being lavished on something they would feel uncomfortable experiencing without little kids.
What percentage of guests have small kids and won't feel awkward on the attraction? Maybe 25%? even 10%? Much lower than for the coasters, IMHO, which most guests can enjoy if they want to. Though sadly, the coasters do divide families, my mom won't ride them and spends time alone while we went on the mountains as kids . . .
If Magic Kingdom had a lot of attractions like Disneyland, then this wouldn't be such a big issue for some, especially since it may be a couple decades until something new is added.
Anyway, that should answer the question of why some guests are unhappy that maybe only 15% of guests are "welcomed" to enjoy this attraction, at least based on the videos of families with little kids . . .