I just don't think they'd drop a season of Mandalorian and a separate season of Boba the same month. I'm not saying they won't have multiple shows on at the same time in the future, but it would be weird to have concurrent shows that are in the same time period, both led by Mandalorian bounty hunters. That would get confusing for casual fans really quick.

I really think that season 3 of Mandalorian IS "Book of Boba Fett" with Din only showing up as a side character within the connected universe.