After thinking about it for a while I think I now understand the true significance of BDSP/Legends. In order to understand what I'm getting at, let's go back to the original Gen 4 games.
A lot of people like to say (sometimes disparagingly) that Gen 6 was the beginning of "modern" Pokemon, but I've always thought that Gen 5 was the true start of that era, for it began so many trends that would define the period up to now, from relatively small QoL things like permanent TMs, deprioritization of HMs and speedier gameplay to major design philosophy shifts like a greater emphasis on making compelling human NPCs.
With all that in mind, if Unova was the beginning of a new era, then the original Sinnoh games represented the capstone on the old one, taking that era's design motifs and bringing them to their logical extremes: The scope and stakes of the legendaries are to this day the highest the series has ever seen, Pokemon from all 3 preceding generations got new evolutions, HMs were in abundance, so on and so forth.
So what does this all have to do with BDSP and Legends? Well, I personally think that we are once again witnessing a transition into a new era, and just like 15 years ago the Sinnoh region will serve as the final note for the old one, except this time it'll also be taking the role Gen 5 once did of ushering in the new via Legends. Game Freak has been pretty explicit about that title being not just a one-off experiment, but a microcosm of their vision for the future of the franchise. This is delving into speculation a bit but I think their main takeaway from SWSH's reception was that developing the same way they did on handhelds just doesn't work anymore on home consoles, and with that in mind we can already see the signs of this in what little Legends footage we have, what with its larger scope and improved battle animations. With that in mind it makes sense to have a game like BDSP to wave farewell to the old way of doing things, especially since as I've explained it's a poetic mirror of the role of the first Sinnoh games.