I disagree, for two reason.I really don't think comparing a Gorilla Tactics ban to allowing underwhelming Megas in lower tiers is a fair comparison. Gorilla Tactics is an ability that is baked into the Pokemon and its design, while Garchompite was an item introduced 2 gens after Garchomp was created. In that hypothetical you presented Garchomp can still use its base form's talents without an item, while Darm without Gorilla Tactics has to bank exclusively on Zen Mode.
- Garchompite may have been introduced seven years after Garchomp, but Diancite was introduced at the same time as Diancie. If Mega Diancie happened to be worse than regular Diancie, it would be very strange to treat it differently.
- Gorilla Tactics might currently be unique to Galarian Darmanitan, but to say it's "baked into the Pokemon" is a stretch. This isn't like Multitype or Shields Down where the ability is intrinsically linked to its sole user. This isn't even like Wonder Guard, which is clearly designed around a single Pokemon. Gorilla Tactics is just... an ability. It's tied to Galarian Darmanitan in the same way Motor Drive is tied to Electrivire or Fur Coat is tied to Furfrou. Execpt it's even less tied to G-Darm than those other two, because it has the option to use Zen Mode.
Also, the more I think about it, the more I feel like Zen Mode G-Darm isn't even worth saving. Unlike regular Zen Mode Darmanitan, Zen Mode G-Darm doesn't change all that much when it changes form. You could swap Zen Mode out for Unburden and be left with fundamentally the same belly drumming, berry munching ape.