Both sides of the argument are fucking idiots.
Yeah it's true it's a little bit silly you can't get the ideal nature but keep in mind this is POKE BANK as in you can just transfer your gen'd or SR'd Celebi from the previous gen if you're so anal about using that piece of shit competitively. If you're so competitive as you claim to be you shouldn't be using Celebi as he's a piece of shit besides being the literal worst type in the game. Then you go and make fun of people for being "autistic" about wanting shinies even when you yourself insist on having boxes full of Smogon-approved natures. Hypocrites.
At the same time there SHOULD by now be another way to change natures or something, but again using legendaries kind of offsets that. Celebi has 600 BST. Does it really matter even if it's neutral natured? So it won't win against gen'd Celebi, whoop de fuck.