There will always be an optimal build and players will always want to find it.
I think this is true to some extent, but it doesn't mean that you can't strive for it. Pragmatically speaking, we know that there will be some hard bound champions, because we can't make double the number of Mythic items due to complexity of the game.
To push back on your point a little bit, I would argue that in a lot of cases, where players think things are optimal, there is actually a hidden build that is competitive, waiting to be discovered. In the best case scenario, the newly discovered build doesn't take over the old build, but is situationally as competitive. We see this in many games, not just League.
To give a League example, the AP items that were changed in patch 8.4 took almost 2 years for Pro players to discover that there were builds other than Ludens -> Morello -> Sorcs on champions like Orianna, Syndra that vary based on the game state. There was a high AP Seraphs Spellbinder Deathcap build, or a more one shot oriented GLP Spellbinder build. We had originally planned these builds out, and knew they were strong internally, but they just took a really long time to catch on; but that's part of the fun of being a theorycrafter and discovering new builds. I don't think you could argue that there was "an optimal" build for Orianna on patch 10.22. Sometimes your composition needs "scaling" via Archangels Gathering Storm, Cosmic Insight, sometimes it needs "spiking power" with Biscuits Scorch, sometimes it needs pick potential with GLP + Spellbinder.
To use a post preseason example, if I was Soraka on Live patch and playing against a bunch of bursty champions, Moonstone might be a worse choice than Locket. By contrast, if I was Soraka playing against a bunch of DPS oriented, but not bursty champions (eg. tanks), Locket may not be as useful and Moonstone would be more beneficial in the longer fights. I wouldn't say one of these options is always better, I truly believe there to be a situational choice here.
Having said all this, I'm not going to argue that we have succeeded in creating choice for all champions, but I will say that we've made a meaningful stab at it and will continue to try to improve on it.