I could only imagine the whiplash the writers would get for that stunt. That would be worse than another Tobias I bet XD
Honestly I think most would be fine with it. I think they just hate seeing Ash lose but if Ash was shown to be the strongest still people would not mind. From what I can see people are just mad when a random guy comes in to beat Ash but I think if a legit reason like Flare doing something went down it would be a lot less hated. Plus it happened in the games (just one gen later lol)
As for a few eps of the final arc I would love an arc with the OG trio just doing random stuff around Kanto. Even if it was all filler would be nice for the anniversary and easy since Ash,Brock and misty would all be in Kanto. I mean I loved the eps in Pallet before Ash went to the league so I would be fine with something in that style. I just want a break between regions and time for Ash to chill over going right to Alola.