I said the same thing about the Grateful Dead. And, on paper, it doesn't make sense: any casual fan will crack open a compilation or best-of or whatever, and the hardcore already have nice copies of everything available, and honestly it's a lot cheaper to build your own box. Someone did the numbers and you could build a mostly mofi version of the GD box for like $100 cheaper. The only real incentive was the one live album that hadn't been put out yet, but that 1 album wasn't going to snag anyone except the obsessed faithful. They really overestimated the deadhead's willingness to spend money for no reason.
But it still sold and apparently made money, so.
I guess a Miles box would live and die on which specific albums are in it. Something early, something bop/cool, maybe an orchestraor soundtrack, a live album probably, an album with a also-famous collaborator like monk or trane or whoever -- miles worked with everybody after all, something from the electric period and one of the latter day albums no one really talks about so you can call it "an overlooked gem" in the press materials.