Ah, yes. Delete the champions that Riot put months of effort into while designing lore, characteristics, gameplay elements, and we also get the added bonus of removing the mains of those champions.
Okay, seriously though: A revert wouldn't be a good idea, especially one as drastic as going back almost 4 full seasons.
as well as reverting some reworks like Galio.
Galio's rework was amazing and well received by most, his popularity also skyrocketed because of it.
And bringing back the older but BETTER rune and mastery systems.
Subjective. I hated having to grind to purchase every rune, I enjoy the uniqueness of our current runes instead of just getting 27 MR at level 18, 9 armor, 3% bonus MS, and 10/15 AD/AP or whatever it was. The old runes and masteries had huge, glaring problems as well.
That's all you have to do Riot. It would give the playerbase a sign of trust if you did, not to mention, you really need to stop thinking about your wallets in the first place.
That's how businesses need to think. Even though some people would love a revert, other players would end up quitting. Your/our opinion of Season 5 being great also is just an opinion, there are plenty of people who enjoy the current state of the game.
Season 5 was my favorite season but it wasn't perfect either, as Pokemonred200 stated before.