Feels like the majority of ARAM games (or at least ARAM players) are played below Plat. Not to mention I doubt MMR is very reliable.
The majority of all games are played below plat. Using plat+ is still a good metric because it tells you what is succeeding in games with players who have demonstrated they have a relatively solid foundation in the game. Also, MMR isn't exactly hard to track. Track wins and losses, compare against others and their wins and losses, and that's it. It's a pretty simple type of formula, particularly for wide generalizations like "who is plat+".
Even going by your precious winrates; there's Fiora, who was buffed, 58% winrate; Caitlyn, untouched, 57%; Urgot, untouched, 57%
Fiora's buff was based on a time when she was at a 48% winrate, and probably could have held off until the poke nerfs went through. However, Cait wasn't winning as much before many other adcs got nerfed, and Urgot got much better after poke went down and frontlines became more prevalent as a result. There will likely be more changes. We're still better off than before the balance changes happened in the first place.
clearly poke isn't exclusively winning.
No, but it's still the best thing to have, and that is despite this being post-nerf. Pre-nerf, Sona and Ziggs had over a 70% winrate. They were the best, unquestionably. All the champions near the top were either poke champions, or champions like Maokai who rolled poke champions. Also, poke is still great, despite the nerfs.
Once poke with a basic frontline isn't blatantly the best strategy, then I'll worry about Riot's "nerf all poke" mentality. Until then, nerf poke more. I want to see more poke champions sitting in the middle winrates like Morg and Zyra (previous members of the top 15).