I think we just have to get used to the fact that movie-franchises are made differently now. They are part of larger continuities, and SW in particular now has a commitment to a larger expanded universe. Lucas may not have cared (though even the PT had EU connections), now however, the EU and the movies are strongly connected. RO tried a milder version of it with Saw, and now Solo has fully committed to it. I think Lucasfilm knows that they can’t do exactly what the MCU does (e.g. making three or so movies a year), so they’ve committed to EU connections instead.

And if this whole Maul thing bugs too many people they can always “fix” it by making him a larger part of a future anthology films. Yes there’s no official plans for it right now, but they’ve opened that door now. I don’t see this becoming a huge problem, especially with younger audiences (it’s easy to forget that the post PT generation(s) actually likes Maul), but if it does they can still “salvage” it in many different ways. Like Silverwook said; Thanos wasn’t know by everyone watching the Avengers (or GOTG, etc.), yet it eventually paid off and those who hadn’t read the comics accepted it. Lucasfilm can still do a similar thing.