Those cons are being exaggerated a bit. It's recovery isn't that bad, with Synthesis and the more uncommon Leech Seed being viable options for immediate and passive recovery. It's not that easy to wear down, with a great ability basically nullifying two of it's weaknesses, and it's other ones can be covered via small support, and that support is almost parallel to the Pokemon in A+. And being scared out isn't that bad, unless the Pokemon is a huge threat to your team. I agree with the pros, though. I am adding to the pros though: the offensive set can 2HKO Ferrothorn with HP Fire, and can take all of it's moves with ease.
EDIT: It can also check Clefable and Mega Altaria, the two most threatening Fairy-types in the tier, and very prominent attackers or stallers in the tier very reliably.
It only gets 8 synthesis, and no other reliable recovery bar leech seed which causes it to lose its coverage option( giving more free turns to its numerous answers) and giga drain only goes so far. When its switching into scalds in the sand, you will see why it gets worn down fast in the current meta. It's bulk is great , its ability is great, its offenses are great considering how bulky it is, and its ability to check lots of deadly threats in one slot is also great. But when you have a host of checks and counters in the meta you will see why mega venu has its struggles. in its defense tho some of its good checks are also annoyed by switching into sludge bomb poisons.
I agree with most of what you had to say and that just because people finally came to their senses and started using Mega Venusaur again does not mean it has to automatically move up in rank. However, I thought a few of your points were a bit unfair regarding both pros and cons.
(Bullet 1, Cons) Saying that its recovery is really shitty for a tank is a bit of an exaggeration. Sure, Synthesis isn't always reliable because it only recovers 25% in sand and rain, but calling it really shitty is an overstatement. Rotom-W has really shitty recovery for a tank. Mega Venusaur's recovery is just not always reliable. I do agree with your second detail of that bullet point which said that it doesn't "blanket check" half the meta like the pro A+ people claim.
(Bullet 2, Cons) Agreed, it has many common checks and counters
(Bullet 3, Cons) For the most part I agree with this point because it lacks Leftovers recovery, but we can't forget that Heatran is A+ and it is the definition of easy to wear down
(Bullet 4, Cons) I've always thought that the support arguement was kinda bs when being applied to rankings and I think that's why AM changed the defintions in the OP and told us to ignore them for the most part. A lot of defensive pokemon such as Mega Venusaur are generally the supporters themselves. It's hard to rank them based on the amount of support they need because many teams are built around one pokemon or a core of pokemon and Mega Venusaur is the kind of pokemon added to support or glue the team together. I get the support arguement when we're talking about ranking something like Charizard or Talonflame because of their huge Stealth Rock weakness, or Excadrill needing a sand setter (which is in A+), but in the case of Mega Venusaur I think we're better off ignoring how much support it needs because it's generally a 'mon that supports others.
(Bullet 1, Pros) I agree that it's sorta an anti-meta pokemon because many people don't prepare for it, although that's starting to change.
(Bullet 2, Pros) I agree with this although like I said before, I wouldn't include support into an arguement for a pokemon such as Mega Venusar to rise/fall
(Bullet 3, Pros) I wouldn't exactly call Mega Venusaur "versatile." I guess it can have Hidden Power Fire, Earthquake, or Sleep Powder in that last slot so people should be careful with their Ferrothorns and Heatrans. And it technically does have two different sets, one being an offensive tank and the other being a defensive tank. However, there isn't really a big difference between the sets and for the most part I wouldn't call Mega Venusaur "versatile", nor use that as an arguement to move it up
All in all I did think this was a pretty fair quality post, especially from a new member.
At the end of the day, Mega Venusaur will be either a strong A or a rather weak A+. Personally, I see it fitting better in A+ for a few reasons that I haven't seen anyone else state (sorry if I missed something):
1. This is the probably the weakest of the three reasons, but I still think Mega Venusaur isn't prepared for enough and is therefore slightly anti-meta. However, I believe that people are starting to prepare for it more, significantly more than a few weeks ago, so I can see if this point doesn't hold up.
2. Mega Venusaur can reliably switch into every Mega Altaria set. Mega Altaria is certainly one of the best pokemon right now due to it's sheer versatility. Is it physcial or special? Does it have Fire Blast, Earthquake, or neither? Can I status it, or will it just refresh that off? Should I try and do chip damage to it as it sets up so I can pick it off with priority later, or will it Roost on me? What if it's running that infernal Cotton Guard set? Well it doesn't matter if you have Mega Venusaur, and I don't think any pokemon in OU can take on all of these sets besides Mega Venusaur.
3. I'm sorry, but I just cannot see Mega Venusaur two ranks below Clefable. Both are excellent glue pokemon, and while Clefable does have a few utility advantages such as Stealth Rock, Thunder Wave, and Calm Mind, I just don't think it's that much better than Mega Venusaur. I get that those advantages might make it better enough to justify a one rank difference, but a two rank difference is just too much. I get that taking up a Mega Slot is a valid arguement, but that hasn't stopped Mega Gyarados, Mega Gardevoire, Mega Scizor, or Mega Lopunny from residing in A+. They all take up Mega Slots and see further competition because they are all offensive megas, meaning they have to compete with themselves and the three offensive megas in S for their rank to justify their position in A+. So as I see it either Clefable should move down, possibly with a mega in A+, or Mega Venusaur should move up.
Solid points overall. However, Clefable doesn't really deserve a drop on that merit. With magic guard its much more resilient than mega venu to hazards weather and status and can hold lefties and has a reliable recovery move, which is really the main flaws with mega venu. Fairy Typing is also a pretty good defensive typing for clef. I think mega venu should remain A.