I guarantee you that I could drop the average Final Fantasy fan at the start of Shadowbringers and they would get their bearings quite easily within the first 5 minutes of the story. You have your world, and now you're in a new one that's in danger. The bad guys want to destroy that world and doing so would also affect your own because they are connected. This, at a basic level, is not something that is beyond their capacity to understand. They can be filled in on the finer details later-obviously this does not hold for Endwalker because it is specifically built upon the end of Shadowbringers.
In terms of characters, all you'd need to tell someone is that G'raha joined you for a few quests once upon a time and that Ardbert and you fought at some point. It's probably best for new players to view Minfilia as anything but the role she played in ARR-letting them be introduced to her as the Oracle of Light is a mercy. As for the scions I directly oppose your opinion and strongly believe that we cannot deal with another 10 years of this cast. You may have started mid Stormblood or Shadowbringers, but I have been here since day 1. Their stories are over, unless something drastically changes in the 6.X patch series. This isn't about attachment-it's about telling a story that won't bore the audience.
I also maintain that the Scions have very little in the way of goals. At least in Shadowbringers Thancred wanted to bring Minfilia back and took on a more paternal role for Ryne later on. However he was utterly bland and forgettable in Endwalker. He may have wanted to save the world for the First to also be saved, but what then? He isn't even a flirtatious womanizer anymore. He's just boring. Urianger basically boils down to "I totally promise not to lie to you guys" and then proceeds to lie to us or give us some lore/exposition, of which better vehicles of delivery exist.
I'm very open about my criticism of Y'shtola. She is little more than a pretty face whose numerous fakeouts combined with her snarky personality have completely turned me off from her character. She may have allegedly cared about the Night's Blessed but her ego and arrogance clearly prevented her from displaying much of that in Shadowbringers. G'raha Tia was arguably one of the more intriguing characters in Shadowbringers but was reduced to a laughingstock when they turned his personality into that of an insufferable hero-worshipper who is written more like a fanfiction character whose purpose is to make the reader feel good, assuming they like him. His goal is to be our fanboy, and I think it's ridiculous. Give me someone like Morrigan from Dragon Age who actively questioned and argued with us about our choices, instead of a yes-man.
I'll be brief with the twins-Alisae is ok but Alphinaud has gotta go. You can't possibly expect people to tolerate 10 more years of his moralizing, but I will conceded that he does seem to have a consistent goal throughout the expansions: regime change. Unfortunately that's gotten old. So you'll forgive me if I'd much rather start 7.0 with a completely blank slate, and support the idea to offer others the options to start at that point instead of continuing...whatever it was that was going on with Endwalker and its own mess in the writing room. The time has come for another Heavensward style expansion, a self-contained conflict in a new geographic region with well-developed characters that are not only distinct, but believable as well. To grow forward in meaningful ways, and tell a new story worth listening to, the past needs to stay in the past.