I also agree with futz's idea of making them recatchable, maybe after beating one or two hard bosses.
It seems unfair that some people are lucky enough to have competitive legendaries while most of us don't. I understand that, as in any other RPG, luck is an important factor; if they were recatchable (maybe once every two weeks), RNG would still be playing an important factor, as it could be possible to get a competitive poke at the fist try, but it would also be possible to only get it at the 20th. However, not having the chance of repeating the capture is way too unfair.
In terms of competitive game, even if everyone was able to get a 20+ iv legendary, it would not be such a big change for PVP, as they are not overpowered pokes and they can be countered. In my opinion, it would actually bring some variety to the actual PVP meta, making combats more fun.
I agree with PRO policy of making legendaries hard to catch, only after a long quest with high requirements. If this was implemented, it would make the possibility of getting a legendary poke even more difficult, as people would have to spend about 2-3 months to get a decent one. It would also bring legendary pokes to its epic place again because, to be honest, the only utility they have nowadays is to use them as HM slaves.