When mindshields became sling-proof, it only downgraded the game mode to revolution with more steps, except worse for shadowlings. In revolution it made sense to make revs harshly curtailed by mindshields because it was a good balance to the fact that revs can very easily recruit because it's merely a click to do so.
Slings don't have this benefit. Conversions are much more laborious on their part. The problem wasn't really that slings 'snowballed' because there's not really such a thing for slings, imo. They're just winning. When cult wipes out security and starts converting people en masse, they're not 'snowballing', they've just removed the only obstacle that all antags generally have. I'd posit that the real problem isn't that slings 'snowball' per se, it's just that they can win very early and very easily by getting the drop on security before security really has a chance to actually react to their presence and the fact that victory can be so assuredly locked down so early in the game makes people not even want to try to fight back and just slip into apathy as we see ascension 20 minutes in.
Maybe a better solution to this is to prevent the potential for an early game locked in ascension by preventing slings from melting mindshields until they have a certain amount of thralls, or requiring one or two thralls to be present for the melting of a mindshield. This contrasts with the current change that pretty much guarantees a crew victory or endless murder bone depending on the ability to mass mindshield. Now if security is captured and enthralled it's often after the crew has had time to actually call out the slings and at that point security should be able to put up a worthwhile and fair fight, meaning that victories are earned rather than quickly stolen like a round start teleporter emag.
If we're going by the original PR that made this change, the problem was that security was getting thralled and causing the slings to 'snowball' (i.e., win) and if the problem is that security shouldn't be able to be thralled, I don't understand why mindshields were made sling-proof, because that goes far, far beyond the original scope of the problem and just seems like the nuclear option rather than the sensible one.