God, this honestly annoyed me quite a bit as a support main, someone who should potentially have Mobility boots in their repertoire of items to consider, yet they're very rarely a serious consideration.
It is completely understandable to nerf Swiftness boots due to their general dominance as a boot choice. But the reason why they're the go to boot choice should you be looking to impact the map as a support or a jungler, is Mobility boots are in a really bad spot.
Why are Mobility boots so crap currently? A bunch of reasons, but the main reasons typically devolve in to how clunky they just generally are. You take even a single point of damage, perhaps a wayward minion as you're running through a lane, and you're crippled for the next 5 seconds, or if you stop to try kill a ward then you lose all your movement speed and leave yourself vulnerable to a degree that no other boot user has to deal with.
Something needs to be done to ensure that Mobility boots actually are a serious consideration, perhaps have it such that you don't fully lose all your MS upon stopping to control vision, or taking something such as minion damage, instead dropping to a +45 MS similar to other boot choices.
Plus the fact that Wind Dragons seem to spawn every single game and, if it's not a Mountain Dragon, it's probably a Wind Dragon, just means Mobility Boots are often redundant.
Even if you have a lot of Wind Dragons, Swifties are still typically useful because of the slow reduction, whereas MBs have no added benefits. They are simply a "Do I want to roam a lot, and are they the best means to that end?". And often, they're not the overall best choice to roam, so they're just not picked at all.
Should Swifties have been nerfed? Hard to argue against it, when they're a staple across almost all roles, it's hard to claim they're not a bit overtuned.
But the disregard for the reasons why Mobility boots are super weak right now and just going "They're not Swifties which is the only reason they aren't generally built"? That's incorrect and obtuse.