That's assuming they even make it to Shadowbringers because they got stuck in Realm Reborn because of, you guessed it, Crystal Tower.
I don't know how it is on other data centers/worlds, but Crystal Tower was THE #1 killer of a sprout's investment in continuing when I was on Dynamis because they either didn't like the idea of being forced into doing 24-man content for any number of reasons, or if they were willing to give it a try, they quickly got frustrated/lost interest when it would take hours to find even just 12 people for Labyrinth of the Ancients unsync. Given that Dynamis is the data center Square-Enix keeps trying to funnel new players into, you can imagine what kind of impression its leaving on them to think that the game is dead and nobody wants to do a crucial part of its mandatory content. MSQ trials were also a constant throttle, including for myself, where I spent 4 months (non-consecutively) trying to get past various ones with Seat of Sacrifice taking a whole month by its self because it simply would not pop on its own, and PFs never had any takers. (And I'm including my attempts to get it done outside of Dynamis too.)
It's the same reason why anyone saying ALL alliance raids should be mandatory just wants the game to die off faster, because there's always going to be "that one series" someone hates or has zero interest in. For me it would be Nier, where if you told me I HAD to clear it to leave Shadowbringers, then I would just never leave Shadowbringers, and would have eventually quit after I got tired of being permanently stuck.
