to be fair, Mega evos didn't warp the meta, more so make it less stale. Besides giving into popular pokemon and giving them mega evos (Blazakin, Lucario, whos megas are banned), or giving a top tier pokemon mega evos (Gengar/Alakazam which are usually banned), Mega evos help low or bottom tier pokemon become relevant. When was the last time you saw a Mawile or Gardevoir or Pinser in any team that wasn't NU? It gives the devs a way to make a pokemon relevant again by removing its ability to use items.

And since its tied to an item, a tourny can just ban mega evos outright if they want to.