I don't think the X Saga is going to be like "must play to understand anything in the present storyline". As others have pointed out, they are very aware that a lot of people won't have played the mobile games that's why everything in the X saga is essentially in like a 'backstory' type of stage (heck even KHx's like biggest finale twist gets unceremoniously recapped and explained in a couple lines in Dark Road lol). And because Sora also don't know anything about the X Saga, there will always be lots of opportunities for the relevant stuff to be explained/introduced to both him and casual players.
The issue I see, aside from just general game preservation stuff and the fact that actually experiencing it will always lead to a fuller and richer appreciation, is that Missing-Link is explicitly meant to be a finale to the KHx Saga, a saga that only really contains like 3 titles and now 2/3 of those are just completely gone and inaccessible in all official ways and the only one left is a movie. Like I sure hope this is just temporary/they have a real solution coming, because like, while I don't presume KHUx+ is necessary to 'get' KHIV and more, I sure think they'll be necessary to truly deliver on a finale to their own stories and mysteries lol???
