Arcanist having the Supervirus trait is irrelevant as ACN is part of SCH. Or are you implying Proshell is a WHM trait and not a CNJ trait? The balance perspective on Proshell is that WHM had a permanent buff (unless someone died then it had to be recast on them which could be costly) but it's on its own weaker than Fey Covenant which was seen as the SCH alternative. Bosses don't often have large scale frequent aoe spell spam where Proshell somehow makes a gigantic lead in damage mitigation, ESPECIALLY not in a scenario where it's a matter of life & death like Supervirus actually is.
There is NO buff or debuff that is as good at full alliance damage mitigation like Virus, NONE. Dragon Kick? Weaker. Storm's Path? Weaker. Sacred Soil? Weaker. Stoneskin? Single target cast. Adloquium? Single target cast (altho SCH are getting teh ability to put the shield in an AoE). Succor? Too weak. You'll also notice several of the already great damage mitigation tools already belong to SCH or will do so (AoE Adlo). Proshell being given to all healers is a balance symptom of "all tools need to be present" which is okay, BUT it's not well thought out when SCH never needed Proshell and it leads to the imbalance that SCH now has every major damage mitigation tool in their arsenal while WHM got nothing new in that regard.
I'm really looking forward to what WHM is getting, I don't think they will disregarded or "left behind", but Proshell is NOT a fair balance without giving all healers Supervirus as well. Another thing of note is the effectiveness on Proshell even on long fights vs. Supervirus. Proshell scales on M.Def so it's weaker on physical dps, medium on casters and most effective on tanks but the grand total effect is still very minor overall but yea as you said in long fights it adds up. BUT on the big damage aoe's, Supervirus will be there short of the boss not being targetable (T9 Megaflare, T13 Teraflare) and the difference in mitigation is staggering.
You also can't force groups to take SMN to make up the lack of a SCH, this is part of why Proshell is being given to all healers, so people don't feel forced to take a WHM. You also can't force Supervirus to only apply to SMN/SCH/BLM because again, you can't force groups to take a BLM either. Especially not when we also consider that DF is not classist, it doesn't assemble groups on any other rules than the specific tank/healer/dps setup each dungeon, trial or guildhest is set for it.
So again, saying Proshell being given out to all is a balance decision to prevent classism is hypocritical as long as Supervirus remains exclusive to SCH. Supervirus is from not only a healer perspective but a group perspective is both mandatory and simply gamechanging.