There's 2 big problems with giving legendary characters bigger roles:
1: Canonically, planeswalkers are a lot more powerful than nonplaneswalkers. It's hard to get players invested in characters that aren't the coolest and most powerful things in your universe.
2: Legendary creatures can't be in every set because they're planebound, so they can't become the iconic, recurring faces of your brand. This was less of a problem during the Dominaria era when most of the sets took place on the same plane anyways, but it's not really feasible now.
Sure, that's all perfectly true, especially the second point, but the discrepancy is certainly there, precisely
because it's so hard to resolve within their current approach. I've definitely seen people express a desire for legends to be more relevant again (or complain that they are overshadowed by planeswalkers), and I mostly agree with that. Though I guess better stories about more interesting planeswalkers and planes would help, too, if they're going to stick to that model.
Re: your digressions:
Planehopping: it's unsustainable. Oversaturation of planes makes each individual plane feel less meaningful. It also gets harder and harder to find new ground without stepping on other planes' toes. We've had artifact world and other artifact world, dinosaur world and behemoth world, wedge plane and other wedge plane.
Brand identity: I think one big problem is that, while we've had multiple planes almost since magic's beginning, the planes always felt like part of a cohesive shared universe. They shared a general tone and aesthetic. You can't tell a heavy, serious story and have pirate dinosaur world because pirates and dinosaurs are cool.
's what I'm sayin', although I think Ixalan isn't a good example, because I'd argue the setting with its piracy and its colonial and religious themes actually would have demanded a more serious story than the farce we actually got.
Diversity: I don't see what the problem is. Like how is having black people in a setting more "unrealistic" than having elves? I don't see how that stuff is immersion-breaking, especially seeing as how the humans in the only plane that we've seen in real life (earth) have all kinds of skin colors.
There is an important difference between the real world and the Magic settings we usually get:
Most planes in Magic are tiny, or at least the part of them that we get to explore tends to be a pretty hermetic and culturally homogenous one, especially since many planes are top-down planes (or "planes of hats" if you want to annoy Maro). [...] If your setting is tiny and has no known connections to anything outside of it, you can't sprinkle in random token minorities without compromising the internal logic of your setting.
The second Innistrad block is the clearest example of blatant tokenism, with minority characters whose presence seems highly implausible. The setting was originally presented as very geographically isolated, with sea travel being extremely limited by the Nebelghast and with people not knowing what lies beyond the Stensian mountains. That premise never changed, but that didn't stop them from inserting a significant number of black people and one or two other minority characters into the set, so you can kinda tell they didn't plan for them to be there the first time around and just included them for diversity points when they returned to Innistrad. Granted, the original Innistrad set had
Grizzled Outcasts, but I'm pretty sure I recall the artist stating he included a black character in the art because he wanted to reinforce the sense of a group of people who wouldn't fit in, so that should tell you everything about how Wizards looked at the setting originally. Or take the idiocy of Chandra's parents supposedly being a black-haired white man and an Indian woman. The plane where the Earth comparison works best is Dominaria, because that one is
actually big, diverse and pretty interconnected, which makes the inclusion of characters from different ethnic backgrounds even cooler because you have a sense of where they or their ancestors are (or might be) from. Jodah, Naban, Naru and Adeliz as well as many unnamed characters are all hanging out at the Tolarian Academies for instance, each of them with a different ethnic background, and it makes perfect sense and even gives the setting more depth.
At the end of the day, all those conflicting forces in Magic are the result of not having Dominaria as the central plane anymore ("central plane" in the sense of getting an even split between Dominaria, new planes and returning planes). Which is really a shame, because the new 'historic' approach and its focus on legendary cards would make it easy to balance a central cast of planebound heroes with planeswalkers hopping from world to world, especially with the
Weatherlight being around again.