What's been bought is the publishing rights to extant songs. So the new owners make money by licensing the music to advertisers, politicians....or by finding new ways to sell the music to us. Since every album has been remastered to death a thousand times, it's difficult to see what they can do in that regard. \
Phil used to keep rehearsal tapes himself, and soundboard tapes from concerts....indeed it is heavily rumoured that some of the better quality bootlegs out there were leaked by Mr. Collins himself as a gift to fans.
I doubt they kept "all the rehearsals". They rehearsed for weeks on end, hours per day....one album might well have produced a closet full of cassette tapes or DATs. Even so, what will exist is still hours and hours of ideas that didn't work, noodling, mistakes, mis-steps and so on.....to have the guys listen to it all and someone actually prepare a reasonable (say four hour) release for one album's rehearsal sessions is quite a task.
What we do know, because it was announced officially, is that two full dress rehearsal shows of The Last Domino were recorded in 4K vision and 7.1 sound, I believe. With the intention that all the effort put into rehearsing the music and creating the stage/light show not go to waste in the event that the tour couldn't take place due to Covid. The raw footage is in the can, waiting to be edited. Though those performances are probably not as good as the latter stages of the tour...even fan videos show that the band was totally on fire by the final leg and Phil's voice improved.
What we don't know is the extent to which those later gigs were recorded. There will be video from the cameras doing close-ups of the musicians for the big screens. But was the light show captured? There will be audio from the sound board but are there full multi-track recordings enabling a thoroughly professional mix to be made? Or would they go back to the dress rehearsals?
I'd love to hear more, and see more from the past tour videos. The Mama Tour VHS is nice but it's not a complete concert. Was the complete concert filmed? Does the original film/tape still exist? We fans might well enjoy something of lesser quality just to see/hear it....but a general release will need to sell to hundreds of thousands of people if it's going to make money....so will need to be top quality.