swarm is a debuff, doesnt’ stack on itself, vines flat dmg does (and ticks twice as fast)
Similar reason we don’t compare BWC to Thermite Mines 
There’s also a potential argument to be had that debuff skills “can’t” be allowed to deal too much dmg “by themselves”. But even such exploitative options aside, it ofc also is a sorta core part of the skill type; it’s a support skill, debuffer, not a main dmg dealer.
If it casually did as much dmg as other main direct dmg/“skills which main or sole purpose is dmg”, it would get kinda inherently out of line snap’ish being selfsupporting then. So killing trash as support is perfectly tolerable i’d say.
And since its primary purpose then is support, the natural question then becomes if that support is fine or too high.
*tho i’m honestly puzzled by Mi0 dissing swarm, as i distinctly recall a convo where “clunky pre-buff swarm” Ritualist was selected for vitality drain essence (a skill that dislikes interrupts) over both OK and Occ, specifically for the sky high vit RR amount 
So it clearly had its usecases for that “pure” support