I disagree, but I also disagree with the transitivity of bans, which is another PR discussion that
happened a while ago that never really reached a conclusion. I believe these topics are inherently connected.
When a tier bans an ability, they are not banning any specific Pokemon. A Pokemon is banned because it has a specific combination of traits that make it overwhelming and difficult to handle. When an ability, move, or general strategy (see: Baton Pass) is banned, we are banning a single trait that makes a wide variety of Pokemon overwhelming within the context of their tier. Going with your example, Swift Swim could be deemed uncompetitive in UU, thus making Mega Swampert unusable, but this doesn't mean Swift Swim is broken in the context of RU, where their different set of viable Pokemon could potentially handle Rain teams better.
However, in a theoretical world where this transitivity was ignored and Swift Swim was usable in UU but not RU, an entire Pokemon would have been banned without having been formally judged for its own unique set of traits. This is unfair because Mega Swampert was never formally suspect tested and was simply banned by association. Continuing this theoretical example, if Swampert were to ever drop from UU to RU, Mega Swampert would be able to fall with it, and RU would be able to judge whether or not Mega Swampert is a healthy or disruptive addition to their metagame. This is how tier shifts have always worked for banning specific traits of Pokemon, such as Quagsire falling after Baton Pass nerfs and Kingdra falling after Swim Swim nerf in BW OU, and it's ridiculous to assume every ability will have the same impact on tiers below them.
If we're moving forward with the backwards system of preemptively banning abilities in lower tiers, I'd agree with banning megas with banned abilities. However, it otherwise makes more sense to keep them in the same tier as their base form so they can follow usage trends in the same fashion.
I understand this touches on issues that might detract from Zarel's main point, so maybe it's a good idea to revive the old thread on this issue.