You say that mid has a big-ass pool of viable champions, yet when assassins were not favoured you'd hardly witness an increase in overall variety with re-occuring picks in solo queue and the competitive scene. If it ain't Fizz / Zed / Yasuo, it'd be just as narrow of a selection consisting out of sustained DPS / poke / whatever mages in their place. Truly riveting stuff. Both 'metas' are just as bad in this regard; the community at large is narrow-minded and suffers from so much selective memory to the point they always deem only a handful (or less) as valid picks, on top of refusing to properly adapt if need be. "Oh man, a snowball champion? Better stubbornly bash my head against a wall thinking a squishy AP mid will work out for me this time!"
Not to say there aren't numerous champions that are legitimately strong in mid lane (or elsewhere), but the community is too pre-occupied with their finger-pointing mentality alongside overblowing the supposed problem cases in any given lane. Especially when we're talking about the god damn solo queue circus. Too much crying about Lee Sin excelling at everything (which he sure as fuck doesn't) for example or wittle Zed being too scawy or whoever else comes to mind instead of looking for alternatives (in a roster with 100+ choices) that abuse their weaknesses. You could argue about how League being in a constant state of flux interferes with this, but extreme exceptions that dominate everything else rarely slip through for an extended period of time anymore if at all, barring sweeping pre-season rebalancing.