Need I remind everyone that just because the "Sgt Pepper" tapes appear to have been checked out sometime in the 70s for Quadraphonic mixing activity does not mean that a full Quadraphonic mix of the album was ever completed, and even if it was completed, it might not actually be that good.
Yes, sometimes certain Quad mixes were held back from release back in the 1970s, and they have since resurfaced to become sonic wonders on the new high-res surround formats (like "Spectrum" by Billy Cobham or "Eli and the Thirteenth Confession" by Laura Nyro)
Other times however an unreleased Quadraphonic mix is not all we had hoped it would be once we finally hear it. A few great examples of that would be the unreleased Quadraphonic mix of "Too Old to Rock 'n' Roll…" by Jethro Tull (which I personally think is an incoherent and muddy mess of a mix) or the Quadraphonic mix of "Echoes" by Pink Floyd (which has so much unnecessary pan pot action in it, it makes me seasick!)
I'm the kind of person that I would prefer them to just release anything and everything and let us (the listener) decide what we want to hear.
(I personally prefer the Steven Wilson 5.1 mixes of the Jethro Tull albums over the Quadraphonic mixes, but I like how they are now giving us the opportunity to hear both mixes of certain albums on the same release.)
However, I know that the Beatles are super anal and particular about what they release and how they want their legacy and their image (and brand) to be preserved. (And that's why we may never see a proper release of the "Let It Be" film until there's only one or no living Beatles left.) So I understand if the Beatles are not keen on releasing any Quadraphonic mixes that don't live up to their own standards.
But finally, in all honesty, I'm not sure many of us will yearn to hear any attempts at a Quadraphonic mix of "Sgt Pepper" if the 5.1 surround mix really is a winner!
Thanks for reading.