I think the issue isn't that it is hard to get bro. The issue is that there is no concrete path which shows "this is what you need to do for mewtwo". With ttar which you lack you know exactly what to do about it-ttar raids. Quite different from mewtwo no? I mean in the main series we only got mewtwo AFTER beating the game as a reward so yeah, make it hard by all means.
However, in this situation Niantic has made it not hard but random and inconsistent. They could have made it plain hard with something like "defeat 600 legendary raids" or something like that. Still a grind, but at least you could see the peak while climbing the mountain