The fact of the matter is that they've been trying to make a third National Treasure movie - with Cage and the original cast - for YEARS, but haven't been able to crack the script. They got close on a draft a while back, that had something to do with some French something or other artifact (I can't remember what my friend who's worked on the movies and read various draft said it was), but point is, it wasn't for a lack of trying.
Once Disney+ became a thing, though, what they *were* able to crack was a "new generation" type series, that had more time to breathe and develop than what the limitations of a two-hour movie allowed. Could they have figured out a way to have Cage's character as the mentor in this series instead of Justin Bartha's? I'm certain they would have loved nothing more. But I'm guessing Cage either had scheduling conflicts (dude is busy as hell with other projects) - or - he didn't want to do TV (at least in a starring capacity, at least not in the first season).
In other words, Cage wasn't "replaced."
Do some of you REALLY think that they ACTIVELY decided to leave NICHOLAS CAGE out of a Nation Treasure TV series, in favor the fourth guy in the Hangover movies and a bunch of young actors no one's ever heard of? Just to score imaginary "woke" points over gaining tangible subscriptions, which would lead to actual financial gain? I swear, the mental gymnastics some of you perform, just so you can act offended on the internet by the "libs" and "woke culture," in the most obtuse, most ill-informed fashion, will never not surprise me.
As for the diversity of the new cast itself, the point, as with all of these things, is to appeal to the broadest audience possible. Which means casting as many people of as many different cultural backgrounds as possible, so as to appeal to a wider, global audience. Because the wider the audience the more $$$ Disney makes, and as gung-ho for capitalism as so many of the anti-woke are, one would think they'd applaud such efforts.
