Personally, I think they should've just made The Mandalorian be Boba Fett. Let him get out of the Sarlacc pit and from there just use the Mando story. As far as actual film lore, we know very little about Fett. His story could've easily just been Mando's. Forget all that EU stuff.
IIRC, from all my reading and discussion with some people in the know, I believe this is what went down:
Favreau's original Mando take WAS for Boba Fett originally, but then he was told by Disney he couldn't do a Boba Fett show because they were planning a Boba movie. So he took all his best Boba material, and built
The Mandalorian around that. Hence all the orphan and "not born of Mandalore" references. Very similar to Boba Fett's backstory.
But then, thanks to major production problems with
Solo, Disney canceled the Boba Fett movie, leaving Favreau in a rough position to try to figure out something new for Boba. He wrote a high-level script treatment for
The Book of Boba Fett, and left the rest in Rodriguez's (ostensibly) capable hands, so that he could go make
Mando Season 3. Rodriguez fleshed out the rest of the scripts under Kathleen Kennedy's tutelage. When Favreau heard that production for
BoBF was going down the tubes (another long story), he came back, killed a couple of the worst
BoBF episodes, and sacrificed his best
Mando Season 3 episodes to be used as filler (all of "Return of the Mandalorian", and some of "From the Desert Comes a Stranger"). Which dramatically transformed How
Mandalorian Season 3 would later shake out.
And yes, the original plan was for Grogu to stay with Luke, kicking off a new Jedi Academy series. But that was too far out for Disney, so they told him they needed Grogu not to go away, in order to save the toy lines. So he was required to have Grogu make the other decision in order to help save
BoBF and keep the merch flowing.
