Physically defensive Volcarona is not a viable Pokemon, and it doesn't stop Mega Lucario from doing its job. +2 Mega Lucario, both special and physical, will KO Volc after Stealth Rock. Same with Moltres. This means that neither of them can switch into Mega Lucario, as it can either attack or set up and kill both of them afterwards. Tentacruel is a terrible check because:
+2 252 Atk Adaptability Mega Lucario Close Combat vs. 252 HP / 240+ Def Tentacruel: 270-318 (74.1 - 87.3%) -- guaranteed 2HKO after Black Sludge recovery
Aegislash isn't a counter, just a solid check, because it takes nearly 50% from Crunch/Shadow Ball/Dark Pulse. Assault Vest Azumarill sounds terrible, and whether or not it is, it's never used like ever. Plus:
+2 252 Atk Adaptability Mega Lucario Close Combat vs. 252 HP / 0 Def Azumarill: 341-402 (84.4 - 99.5%) -- guaranteed 2HKO
+2 252 SpA Adaptability Mega Lucario Flash Cannon vs. 252 HP / 4 SpD Assault Vest Azumarill: 294-348 (72.7 - 86.1%) -- guaranteed 2HKO
Again, not a counter. There are a lot of soft checks to Mega Lucario, but it's really hard to outright counter, especially without knowing which set it's running. SD is horrifying because it can spam Adaptability Close Combat against offensive teams and set up and sweep against defensive teams, and NP is scary because it has coverage that keeps dedicated Fighting resists from owning it.
Also HP Ground Genesect is a thing, albeit a rare one, that's used in order to lure in Heatran, usually for something else like Mega Charizard X or Talonflame to clean up later in the game.
While this wouldn't surprise me, I don't think it's the case. The whole point is that you come in either against something slower (there's still a shitton of mons who are slower than +Speed base 90), something faster that you can pick off with priority, or something you can set up on. It's really not hard at all to get Lucario mega evolved.