I disagree with these points completely. Yes, you are always given the "main" legendary from each version as part of the story, although capturing it is rarely vital, but the others are still optional sidequests, some of which are far more difficult to locate than they were in the original games. Look at the Regi trio for example, which involved working out a series of fairly complex puzzles by deciphering clues that were only given to you in braille. Each one of the recent games have also had a set of Pokemon that "run" and can only be found by specifically cornering them into sections of the map (or by random chance).
Mewtwo may have seemed more difficult to obtain back in the day, simply because the internet was still in its early days and the locations of Pokemon were only passed around playgrounds as idle gossip, along the lines of "I heard if you do this and then go to....". Even then, it's not like kids couldn't figure out how to find Mewtwo or at least one of the birds. I don't think there was ever a time when Legendary Pokemon were uncommon and the ones like Mew, Celebi and more recently, Vicinini, are still hard to come by, due to the fact they rarely distribute them.
If anything, it's harder to locate them now (ignoring ones that have story points centred around them) and there's even a lot of non-legendary Pokemon these days that can only be found by evolving others in very specific ways, during specific seasons/days or in certain hidden areas. There's a lot more complexity and nuance involved, the fact the information about them is so much easier to come by is not the fault of the games.