I thought making sequels but with major inconsistencies was generally considered a sign that the sequels were or were becoming increasingly cheap, even embarrassing.
You can't generalize about such things. There are many reasons for such inconsistencies. For instance, in the case of
Planet of the Apes, none of the first four films was intended to allow for a sequel, so when they were successful enough to warrant sequels, it was necessary to retcon their assumptions in order to make those sequels possible. Sometimes a movie series (like
Highlander or
Halloween or, heck,
Superman if you count
Returns) disregards one or more failed sequels and pretends it/they never happened.
But the main reason is that your highest priority when you tell a story is the good of
that story. The goal is to make your current project as good as it can be. And if that means you have to reinterpret or ignore something from an earlier story, then that's what you do. Because these are not encyclopedia entries, they're colorful lies for fun. It's nice if they fit together convincingly, sure, but since they're all just pretend to begin with, it's not a higher priority than the quality and enjoyability of each individual story.
My biggest issue with the newer X-Men films is that in FC and DoFP (aside from them having weak/uninteresting writing in general) Xavier is depicted as way too flawed, not really inspiring at all, while Magneto is depicted as much more likeable, much more the one to root for, but still pretty cartoonily evil/bad, I think both of those were too disappointing depictions compared to their predecessors. So I'm not really interested in seeing what happened to them later.
They were supposed to be different when they were younger, so they'd both have character arcs over the series. Xavier starts out as an egotistical, immature jerk, but learns his lessons the hard way and grows into the wiser, kinder figure he eventually becomes. And Magneto needs to start out as a more redeemable, morally ambiguous character who can be a friend and ally to Xavier prior to their eventual falling out. Though maybe the sequels didn't handle that arc as well because of the need to somewhat reset the two characters for each successive movie.
