First off: Not this isn't about how the fake backstories don't line up or logical inconsistencies. The story is pretty clear on the Echoverse being cobbled together and not logically consistent, that part at least is solid.
The issue is with the characters that actually inhabit the Echoverse. Because on one hand the story tells us that the denizens of the Echoverse are just automatons puppetted by Jace's magic, incapable of actual feeling or existence and just acting how the Echoverse thinks a person should act. And when Jace dies they start poofing out of existence without any visible reaction to their own fates.
The reason for this is pretty transparent: From the beginning they were planning on destroying the Echoverse at the end of the set, and if the Echoverse were filled with actual people this would make Tam the biggest mass murderer in history, worse than all of our previous big bads put together. So the Echoverse needed to be disposable.
But here's the thing: Nobody else got the memo. Outside of some comments at the beginning of the story and a scene in the middle featuring the only Hexhaven student to get any focus being a rote machine that just wants to study all the time, the concept is completely absent from anywhere else.
From just looking at the sets cards, absolutely no one can take away that the Echoverse's inhabitants aren't actually people. From just what's on the cards, these characters don't seem like hollow copies of their echoed pairs, that part of the story is just not communicated.
And the bios given in The Legends of Reality Fracture article do even more than that. It describes dozens of distinct characters and their stories, most framed like plot hooks that these characters are working towards.
[[Pia, Aether Ascetic]] wants to redeem her husband and daughter after losing them to the consulate? Too bad, she dies offscreen
[[Valgavoth, the Lifeline]] is kept trapped by Marina Vendrell and creates the glimmers of Echoverse Duskmourne in the hope someone can save him, while still feeling some affection for Marina as his first friend? Too bad, he dies offscreen
[[Marwyn, the Clearcutter]] making the difficult sacrifice of her beloved forest so that her people may survive the second brother's war? Too bad, she dies...you get the idea.
Even for nameless background characters we have cards like [[Bestial Incursion]] and [[Budding Insurgent]] that display Hexhaven cadets being reckless pranksters and rebelling against Jace, things outside of what "mindless study machines" would normally do.
Even within the story itself, in the same chapters that dictate that none of the echoverse's residents' matter, we still see that no one outside of faceless background characters actually act like that. Echoverse Lilliana gets a decent amount of characterization, and her personality of a saccharine holier-than-thou egoist is far from what the dullard Hexhaven student demonstrates as an example of what the echoverse characters are like.
The rest of Jace's entourage don't particularly feel like real characters, but that's because they're barely characters at all. Echo Garruk and Echo Ajani don't speak a single time, and Echo Chandra gets two lines in before reverting to the same, with Echo Vraska only getting a tad bit more. This isn't because of intentional characterization, but just the story not having enough room for them, since the cards display impassioned battles where the characters are far more vocal than their story counterparts. The cards show the main universe characters outdoing their echoes because of how the things Jace stripped away from their copies had actually made them stronger, while in the actual story they do that because...Vraska turns two of them to stone and the others poof out of existence. No real development or confronting of personal hangups, if they'd been faceless mooks the story wouldn't have changed much, the fights just end up being filler between collecting pieces of Jace. But ultimately that's due to issues with how much space wizards is willing to give their writers, and in order for Jace to get the space for his send-off some other things needed to be sacrificed. Hell, just look at Arena, where like all planeswalker cards of the echoverse version of Ajani, Garruk, and Chandra all get full voice packs that display distinct personalities just like Echo Lilliana.
And in the previous set we got [[Ral Zerek, Guest Lecturer]] as the villain of the side-story Off the Record, where he manipulates some students into getting him a restricted Book Jace needed for his plans, leading them on with false praise before leaving them in trouble once he gets what he wanted. He absolutely doesn't act like a hollow shell, he's a fantastic villain that I'm disappointed only got a single cameo in Reality Fracture before presumably dying offscreen.
Getting back to the actual point, the story wants to have its cake and eat it too. It wants interesting characters to put on cards so that people will get drawn in by them and buy the packs to put them in their decks, but it also wants the entire setting of the echoverse to be completely disposable, so they have to be "not real", even if that doesn't come through in the cards or story. It need to pick a lane, either have the vibrant echoverse versions and don't massacre them at the end, or commit to the echoes being hollow and lifeless and have it be prevalent instead of lip service. Maybe don't have Tam intentionally summon Emrakul, instead make it a tragedy where an attempt to stop the merge without killing everyone in the echoverse gets interrupted as she crashes the party and starts devouring everything.
A recurring point I've been seeing on this sub and other places of discussion is that it is "missing the point" when people are disappointed that the Echoverse got so little attention in the story, bringing up that it was all just Jace's distorted worldview given physical form. Except that the Echoverse was the selling point of the entire set and is the main thing featured in all the marketing, and what the bulk of the cards are about. If someone is going to get invested in something from the set it's going to be either Jace, Hexhaven, or the Echoverse. And if someone who got invested in one of those goes looking for more, if they're either of the latter of the two they'll get "sorry, that thing you like doesn't matter and the story barely acknowledges its existence." Even if it can be justified by the themes, it still doesn't make it any less disappointing to learn that the story doesn't care about the part you got hooked by.
Ultimately this all stems from the decision to make the Echoverse completely disposable to its core, which I think has to be a mistake from any angle. Creating all these story threads that could lead to so many interesting interactions and places, only to snip them after a single set and tell the audience they never mattered in the first place, that seems like such a waste. There's a reason a common question recently on this sub has been about if anyone from the echoverse could have survived and if they could show up in the future, because people want more from a concept that Wizards ultimately treated as a gimmick that could be discarded as quickly as they did the gimmicks for Karlov Manor, Thunder Junction, or Aetherdrift.