Why does a champ get reworked?
Because they're too dominant in pubs but nowhere in pro play. (Or vice versa)
Why is the pro scene so cluttered with similar picks?
Because Riot makes consolation buffs/nerfs to make sure champs can still be played in pub play.
Why are champs like Kalista, Azir and more considered design failures?
Because they can't fall on the spectrum of "Easy for anyone but hard to master" and are useless in PUB play yet dominant in pro play.
We're rapidly approaching the point (If we haven't already hit it) where Riot will need to make a call. To try and balance a game for bronze-to-gold or diamond plus. This half-measure of constantly trying to balance for everyone isn't working. We're already seeing the cracks with all these champions.
Almost a decade. We had a good run, but you can't keep playing half-n'-half here Riot.