Honestly I think a lot of you are really underselling Talonflame. Yeah the loss of priority sucks, but 126 speed is still absolutely fantastic, not to mention Flare Blitz / Brave Bird coverage is wonderful, and as A-Wak proved fire types CAN be successful in OU with the right support. Not to mention it outspeeds Greninja that aren't carrying Water Shuriken (which doesn't matter if you're full health but is still worth noting), and can check (and some of these it can counter as well) Celesteela, Tangrowth, M-Hera , Jirachi, Kartana, Non-Scarfed Keldeo, Magearna, M-Maw, M-Venu, and Volarona.
126 Speed is,indeed, "absolutely fantastic." You know what isn't? 81 Attack. Don't even mention Adamant since Talonflame still only hits 287 Attack and you're contradicting your own point about its Speed if you run Adamant. Alolan Marowak's success comes from its raw power, Ghost STAB, and useful abilities, and beyond a Fire typing it shares nothing with Talonflame, so why the hell are you comparing the two? You bring up Alolan Marowak and not Volcarona? Heatran, which destroys Talonflame? Mega Charizard Y, which actually
does shares the same typing? If Speed is the only reason you're using Talonflame, why not bring the likes of a Tapu Koko - which, by the way, handles Talonflame quite nicely. You would also never switch Talonflame into a Greninja, unless you really did want to die.
Offensive Celesteela lives a Flare Blitz and OHKOs with Supersonic Skystrike.
Tangrowth itself is supposed to be a
defensive blanket check and Talonflame is far from the only thing that beats it.
Mega Heracross wins if it predicts the switch with Rock Blast or Substitute - not hard, by the way. Oh, did I mention Close Combat kills Talonflame after Rocks? It does like 62% min.
Jirachi, again, is primarily a defensive Pokemon (that
gets Stealth Rock) unless it's Scarf, and Talonflame can still lose to it.
Keldeo's best set is Choice Scarf.
Magearna has this nifty move named Thunderbolt.
Mega Mawile at worst 2HKOs Talonflame with literally any move bar Fire Fang.
Mega Venusaur still exists?
Volcarona beats Talonflame if it's already set up a Quiver Dance and Talonflame is not at full health; both Z-Fire Blast and Z-Psychic OHKO Talonflame.
Talonflame is a shaky answer to the mons you
yourself listed, and let's not get started on its own checks and counters.