I think if anything you don't understand my statement. So I will make it easier to read.
Every Mega has an opportunity cost.
Some Megas like Mega Lucario can easily overcome many of its opportunity costs.
Being in A tier does not mean Venusaur doesn't overcome a lot of the opportunity costs. Rather - it is still a very questionable choice for a Mega. And it is. When you have offensive brutes like Mega Pinsir and co. it is hard to completely erase Mega Venusaur when other walls like Mandibuzz and co. do not necessarily have such a steep opportunity cost behind it.
Coming from a person who used Mega Venusaur as a teammate with Gyarados Mega Venusaur is awesome. But it still can deny other potential Mega Pokemon that are offensively capable of crushing teams.
In short - Mega Venusaur is great, but a tank costing a Mega slot can be hard to justify when there are tanks that can do as well against variant threats without such a high cost associated with them.
While I do agree with some of your points, I suggest that we stop including the opportunity cost of Megas in arguments particularly in placements, the reason which is because smogon has no clear-cut definition on how we should handle this new mechanic, therefore we'll lack a common median and invalidate any argument made.