You guys talking about a reboot be it soft or hard are on the right track. As is the idea of shifting the target audience back to be 10-12,that will hopefully make the show more palatable to older fans.
Here's my compromise idea; Ash after winning in Kalos goes back to Pallet, meets Gary, and Gary drops that he's going to Poke Hawaii both to do research and to test himself against the gyms. The two quickly get competitive about it and decide that they'll both bring out the big guns in order to beat the other to the punch. Both bring back a full squad of favorites but liberally make use of swap ins from back home, constantly cross paths for explosive battles using both old and new Pokemon, and the show hops between Gary and Ash while mostly skipping or cutting fluff episodes, leaving them as implied. They should both be notably older now, late preteens to early teenager ideally. Ash has to stop being sold as some static blob to market the Anime with. Desperately so. There's no permanent growth, so people don't get attached and thus drop the show once the things they liked and stuck around for end, it's as simple as that. Cut some of the fluff, bring back some nostalgia, and play with the delivery of the plot.
I will say tho, Gary is so overhyped it hurts and I bet his image would break within a few episodes of the above.