Does Flare Blitz hit anything notable? Ferrothorn, Scizor and Genesect are terrible in a meta with Primal Groudon everywhere, Mega Metagross and opposing Solgaleo are both OHKOed by a +3 Earthquake, Shaymin-S is OHKOed by a +3 Sunsteel Strike. I think Sunsteel Strike, Earthquake, Stone Edge / Wild Charge gets the best overall coverage.
The first thing that comes to mind is if PhysDef Solgaleo is ever a thing, in which case it does this:
+3 252 Atk Solgaleo Flare Blitz vs. 252 HP / 252+ Def Solgaleo: 466-550 (97.4 - 115%) -- 87.5% chance to OHKO
+3 252 Atk Solgaleo Earthquake vs. 252 HP / 252+ Def Solgaleo: 292-344 (61 - 71.9%) -- guaranteed 2HKO
It comes out as it uses the one Splash it gets and proceeds to PHaze it out with Roar if you lack Blitz. It also requires significantly more weakening to be put into range of 2 unboosted Flare Blitzes than it does to put it into range of 2 unboosted Earthquakes, which hurts in the early/mid game
252 Atk Solgaleo Flare Blitz vs. 252 HP / 252+ Def Solgaleo: 188-222 (39.3 - 46.4%) -- guaranteed 3HKO after Stealth Rock and Leftovers recovery
252 Atk Solgaleo Earthquake vs. 252 HP / 252+ Def Solgaleo: 118-140 (24.6 - 29.2%) -- 6% chance to 4HKO after Stealth Rock and Leftovers recovery
The other one is this:
252 Atk Solgaleo Flare Blitz vs. 248 HP / 252+ Def Skarmory: 156-184 (46.8 - 55.2%) -- guaranteed 2HKO after Stealth Rock
252 Atk Solgaleo Wild Charge vs. 248 HP / 252+ Def Skarmory: 116-138 (34.8 - 41.4%) -- guaranteed 3HKO after Stealth Rock
You don't want to burn your Splash too early, so this has utility before you reach a point where you are ready to set up in that it means Skarmory doesn't come in and outlast it between Roost and recoil while discouraging setup in front of it due to Whirlwind.
The other thing is that you seem to be making assumptions about what will/won't be relevant in gen 7. Sure, maybe in ORAS these Pokémon aren't relevant, but who's to say that they won't be in gen 7? The game's not even out yet; we don't know what trends there will be with a set of brand new 'mons, and maybe they will come into favor due to a good matchup vs. one of them (e.g. Genesect vs. DeoLele).