Well, if we're to take the Global Link stats as an indicator.
I'd actually say that the addition of Landorus, Suicune (far more popular on cartridge than Showdown), and Mega Salamence to the format is what rendered Blaziken obsolete, and in addition, I don't see how it was ever able to get past Gale Wings Brave Bird, bulky Garchomp variants, Choice Band Aqua Jet and the like, but I'm a late starter on this gen so maybe someone can illuminate that experience.
Landorus-T had a similar effect on Lucario in cartridge matches, and all Luc's popular moves are walled by Talonflame.
Swords Dance/protect/HJK/Flare Blitz
+2 252+ Atk Mega Blaziken High Jump Kick vs. 0 HP / 4 Def Mega Salamence: 216-255 (65.2 - 77%) -- 18.8% chance to OHKO after Stealth Rock
252+ Atk Mega Blaziken High Jump Kick vs. 240 HP / 0 Def Azumarill: 164-193 (40.8 - 48.1%) -- 67.6% chance to 2HKO after Stealth Rock
252+ Atk Mega Blaziken High Jump Kick vs. 0 HP / 4 Def Talonflame: 179-211 (60.2 - 71%) -- guaranteed OHKO after Stealth Rock
252+ Atk Mega Blaziken Flare Blitz vs. 0 HP / 4 Def Talonflame: 165-195 (55.5 - 65.6%) -- guaranteed OHKO after Stealth Rock
252+ Atk Mega Blaziken High Jump Kick vs. 252 HP / 252+ Def Garchomp: 204-240 (48.5 - 57.1%) -- guaranteed 2HKO after Stealth Rock
+2 252+ Atk Mega Blaziken High Jump Kick vs. 252 HP / 252+ Def Garchomp: 405-477 (96.4 - 113.5%) -- guaranteed OHKO after Stealth Rock
252+ Atk Mega Blaziken High Jump Kick vs. 252 HP / 252+ Def Suicune: 178-211 (44 - 52.2%) -- 77.3% chance to 2HKO after Stealth Rock and Leftovers recovery
+2 252+ Atk Mega Blaziken High Jump Kick vs. 252 HP / 252+ Def Suicune: 357-420 (88.3 - 103.9%) -- guaranteed OHKO after Stealth Rock
-1 252+ Atk Mega Blaziken Flare Blitz vs. 252 HP / 252+ Def Landorus-T: 129-153 (33.7 - 40%) -- guaranteed 3HKO after Stealth Rock and Leftovers recovery means it can switch in once with SR up and then its done if Blaze sds later in the match.
As for Lucario, again talonflame cannot check it.
SD/Close Combat/Ice Punch, Iron Tail or Crunch/Bullet Punch or Extremespeed was the standard physiucal mega luke set for ou iirc. Dark Pulse/Flash Cannon/Nasty Plot/Aura Sphere was also common
252 Atk Adaptability Mega Lucario Close Combat vs. 0 HP / 4 Def Talonflame: 187-221 (62.9 - 74.4%) -- guaranteed OHKO after Stealth Rock
252 Atk Adaptability Mega Lucario Iron Tail vs. 0 HP / 4 Def Talonflame: 156-184 (52.5 - 61.9%) -- guaranteed OHKO after Stealth Rock
Also Extremespeed bypasses priority brave bird should Lucario run that.
-1 252 Atk Mega Lucario Ice Punch vs. 252 HP / 252+ Def Landorus-T: 184-220 (48.1 - 57.5%) -- guaranteed 2HKO after Stealth Rock and Leftovers recovery
Also the special set slaughters Landorus.
None of these checks and counters are able to switch in with like ANY prior damage, plus they are all stupidly easy to wear down.