Give it a rest already.
There's was no way to give the prequel characters a 'reunion' in the OT because there was nothing for them to reunite from when the OT was made.
the ENTIRE point of wanting a reunion for the ST was that it was the OT that made it ALL possible. There was no PT in anyone's minds (excpet george, to an extent) at that time. The OT made the entire thing possible (no OT, then no PT and no ST), and you get all the actors back and just say 'meh, f it, no need to ever put them together'. It was a purely concsious decision since the killed of han in TFA. And one that makes no sense.
But in the simplest terms, you can't reunite a group in 1977, 80, or 83 with a group that didn't actual exist until 99. Given that, no one cared one iota to see that reunion in the OT. There was no reason to. Those characters didn't exist for them (or anyone else) at that time.
As for as the ST, the original cast was flat out USED to sell tickets. Given what they did to them, they'd have been substantially better off to just move it down the line 30ish years and say they all passed before this kicked up and lived happy lives. Their existence in the ST serves nothing plotwise, really, other that supposed fan service, which is back handed at that.