I disagree with you about Mega Venusaur to be honest. It only handled Mega Lucario if it ran Earthquake, and it's not like that OHKOed it anyway. +2 Flash Cannon, on the other hand, still 2HKOed. Also, the fact that Genesect is banned too probably makes it better. It doesn't have to tank U-turns, boosted Flamethrowers, or allow Genesect to set up Shift Gear or whatever. Mega Venusaur still and will continue to wall the majority of the metagame with next to no direct support. It also has a gigantic support movepool to work with and great power and coverage for a wall. It's just one of those Pokemon that you can basically slap on a team and it will easily help you check and counter a slew of threats, while still hitting back hard and being a nuisance with Leech Seed and the like, so you're not losing any momentum. Therefore, it's S-rank in my opinion.Venusaur needs to be moved down to A+ rank. Being able to handle Mega Lucario was one of the biggest selling points to using Venusaur, and now that's it's not around it's certainly not S rank in my opinion. I just don't think Venusaur is metagame defining to be S rank material.
Rotom-W needs to be moved up to S rank. You can just slap Rotom-W on nearly any team and it will always do well, just because it checks so many dangerous threats like Talonflame and Pinsir. It also offers basically any team great utility in the form of Volt Switch and Will-O-Wisp. Right now, a big percentage of the metagame is all about removing / weakening Rotom-W for your Pinsir or Talonflame to sweep, and keeping Rotom-W healthy is often the difference between winning and losing a game.
That's all I really have to say so far about S / A rank Pokes.
I also completely disagree with Rotom-W being in S-rank; I personally find it absurd to sit there. It's way too overrated and is getting too much attention for its ability to handle specific Pokemon, such as Mega Pinsir and Talonflame. I get that it can do this. I get that it can burn things with Will-O-Wisp. I get that it can maintain momentum with a Volt Switch. However, it's fairly easy to take advantage of with the likes of Substitute Kyurem-B, Clefable, the Latis, and even Suicune can set up on it. And hey, if you're saying that the departure of Mega Lucario made Mega Venusaur worse, than I can say the same with the departure of Genesect making Rotom-W worse (it was a great check to it). Also, it's not like it has access to reliable means of recovery (Pain Split only takes you so far), making it easy to wear down. I don't know, I just don't see it as such a meta-defining Pokemon unlike other Pokemon in S-rank. It should stay in A+ because it is really good, but I can't agree that it's this exceptional Pokemon that everyone needs to be afraid of.