Well, maybe you're fooling yourself. Putting a different power alongside Serum might follow the letter of the Cottage Rule, but completely ignores the spirit of it.
If those two IOs that Mercs can't use are so powerful that the set is strictly inferior for the lack of them, the IOs are probably the outlier and should be nerfed. And I don't think anyone wants the IOs nerfed.
Actually I think you're unclear on the spirit of the Cottage Rule. The purpose of the Cottage Rule was always just to not leave existing players feeling bait-and-switched. As long as the original power remains and is not taken away from players who liked it, the Cottage Rule is respected in both letter and spirit. It never had anything to do with balancing.
And what is the difference, exactly, to a player who wants Serum but knows it's bad, if the devs simply give up on it and put another power in its place, mutually exclusive with it? If that's the approach they take to add the utility to Mercs that Serum is failing to provide?