Tbg...You are quite correct that matrix multichannel from vinyl rarely produced good effect. I think that two reasons are:
1...The rear channels were predominantly vertical groove modulation which is noisy.
2...Four channels from two is too much to ask. Matrix 3-channel (to drive a center speaker) is good.
With CDs, reason 1 does not apply, but the results are still poor. But, with digital, the oportunity for discrete multichannel exists, so why bother with matrix?
.