Also I wanted a series, could have been a 1 or 2 season arch and then Netflix or whoever could flail off into other characters and deeper cannon and non-cannon lore, and I would have had my fill and moved on.
Wicked was published in 1995. Making this movie might have made sense up to 2009, but now? It's a sentimental cash-grab. Same reason they're making Gladiator 2.
The musical premiered in 2003 and has been wildly successful. It's the third highest grossing musical behind The Lion King, which was already and has since become a film again, as well as Phantom of the Opera which had a film made several years back.
This film was always going to be made. Musical films have an iffy track record. Some do great, some do terrible, few are just sorta in the middle.
You can call it a cash grab if you want but even that seems ridiculous. It's a film that was always going to be made someday, it's been 20+ years since the musical premiered, and they've spent a good amount of money on it. The only point you have in favor of it being a cash grab is the fact that it's a two parter. But that's not a cash grab it's just greed.
As for Gladiator 2... I have no idea how that relates to this film.