There are a lot of tropes and ideas that D&D doesn't support or that I would not want to run. There's a My Little Pony D&D crossover, doesn't mean I want to run that campaign nor would I do a good job at it. The youngest kids I DMed for were all nieces and nephews; they're pretty mouthy so if it was an issue they would have told me.
I've found that most young people have plenty of imagination and they aren't obsessed about one particular concept.
If they do want to play some other concept that I don't want to support they can always DM themselves or find a different DM. You can't please everyone, if the DM doesn't have buy in it won't be a good game.
Sure there are things DnD can't handle. There are also a lot of things DnD can handle. If you don't even give an opportunity, how will you know which it is?
I don't get this obsession with claiming that people who are inspired by a concept and eager to explore it somehow lack imagination. I mean, people telling me this would be like me saying "Why is it a big deal that Firefly was canceled, are you so unimaginative that you can't like other science fiction shows?"
Just because I have a third and fourth favorite thing, doesn't mean that not getting my favorite thing isn't a bummer. Just because I finally want to try out a concept, doesn't mean I lack imagination.
And, back to this other oft beaten horse. Your answer here kind of just proves Minigiant's point. If there are players who are seeking things you aren't allowing, they either need to be forced to DM (meaning they can't play their concept) or they need to find someone else. And finding a DM is not as easy as you seem to think in some places.
So, again
1) DnD is big enough to cover a lot of concepts, not just one or two
2) Stop accusing people of lacking imagination.
3) Having a other options doesn't mean it doesn't suck not to get my first choice
4) "Just DM yourself if you want to play that character" misses the point and is again, not real solution.