Anyone can do calculations without context.
First of all, Swords Dance still exists you know. Blaziken is primarily a sweeper, not some designated Baton Passer. The same calc deals 82.41 - 97.52% at +2. That's a 62.5% chance to OHKO with SR, and considering how MegaSaur lacks Leftovers, chances are it'll have some residual damage. Only in lalaland is MegaSaur a check to Blaziken.
Venusaur I concede can check some Tyranitar variants. However, the Choice Band variant is not one of them. On paper, it looks fine and dandy, but then you realize MegaSaur runs no Speed, and sometimes even runs a -Speed nature. Assuming neutral nature an no investment from MegaSaur, Tyranitar can simply run 156 Speed EVs to outspeed it. 76 Speed EVs is a BW2 relic, and if hypothetically MegaSaur is top 30 OU or something, it will convert to the new spread. To be a check/counter, Venusaur has to be the one switching in, taking a hit right off the bat for at least 46.97%. Without Leftovers, Sand Stream tacks on another 6.25% toll, leaving MegaSaur at 46.78%. In this hypothetical scenario, we're assuming MegaSaur is relevant and therefore CBTar will run the EVs to outspeed it, but hold on, the next Stone Edge is a KO! Even factoring in Stone Miss, MegaSaur has a 64% chance to bite the dust before it even moves, blech. I'm too lazy to go in and do Assault Vest Ttar calcs, but I'd assume it'd just be two Pokemon bouncing weak moves off each other for a 124893HKO or something.
Venusaur>Azumarill 1v1, but CB Azumarill's job is to hit and run. Over the course of a game, eating repeated CB Returns is not fun. No competent player would even try to set up Belly Drum Azumarill until it's counters are disposed of.
I do like MegaSaur's matchup against Rotom-W, but there are ways to deal with the washing machine without giving up your mega slot.