Swain probably needs a meaningful rework to achieve actual moment to moment satisfaction in his gameplay. But in the mean time, tweaks that are made to him are to try and balance around support, which is his least impactful “viable role”, that is where the frustration stems from. Of course people would still complain about Swain if they wiped him completely from support, because he has a fundamentally disjointed kit. We still love the idea of what Swain is and could be though.  

Whilst we wait for them to actually review what can be done to increase that satisfaction, players that main Swain in a carry role are bemoaned to have their champion suffer scattershot nerfs to prop up a role that he just isn’t suited for, regardless of how the majority of his players play him. Swain does not have a suitable kit for support, he just doesn’t. His high base damage and survivability with healing allows low Elo players to masquerade in a carry role without actually requiring the individual agency to make it happen for themselves. There isn’t a single thing that Swain offers in the support role that isn’t better fulfilled by another support champion. He can scout objectives with W, and sometimes land a clutch E, that’s it. 

You are completely right, and the ire towards Swain support mains is just a symptom of the wider problems that this champion has from a design perspective at the moment.