Cant have it both ways
Why are people getting super agro about not getting shadow claw gengar on a raid day when they were doing the exact same thing BECAUSE Zapdos got TS on its raid day.
They cant have it both ways, getting mad about a new moveset outclassing a previous version of a pokemon, but then those same people getting mad when that exact same thing DOESNT happen.
Answers
I'm curious why some moves go legacy in the first place. I understand the first wave of legacy after a month because of main series incompatibilities and if new moves are added that are strictly better I can see that too. What do they gain from making viable moves legacy? Did they deem SC Gengar "too good" for the time?
Hard to see how it could have been too good because there were no targets for it. For one, TDO was the king in the old gym system, not DPS, and Gengar was resisted by the 2 tankiest gym defenders while being SE against none. The difference between SE and neutral damage was smaller back then which made Dragonite a borderline broken attacker, but Gengar wasn't anywhere near Dragonite.
I used my SC Gengar a few times in gym fights just for fun before raids came around, but it definitely wasn't useful or anywhere near good.
All the more reason to wonder why viable moves go legacy at all. Especially in cases where the replacement move offers nothing over the original such as Ice Beam -> Surf for Lapras. If I could ask one question directly to one of the Niantic employees in charge of laying out moves this would likely be it.
IIRC shadow claw was mediocre UNTIL they made it legacy for Gengar. IOW they boosted it after Gengar could no longer get it. They did the same with Body Slam after Snorlax could no longer get it.
Easiest solution for Niantic at this point would be to rebalance moves in a way that emulates the Draco Meteor/Outrage situation so that the biggest difference between moves would noticeably be their energy consumption and firing times instead of DPS. If anything, longer bar moves should be 1.5 times more powerful than they currently are to make them worth the risk of even having them on anything.