Nope. Not unfair to us at all. It took me the better part of 6 months to get Moe as a merc, but I already have him now, so all that time will forever be lost. Look into the "Sunk Cost" logical fallacy.
I could care less if new players could pay 100 diamonds to fight him instantly.
I have nothing to lose from that. Actually, I have more to gain from that, since I could then reset and it would not take me another 6 months to complete.
It's something that has been brought up before though, and the devs have not said it's something that is planned for the future, so I don't know if it will happen.
What I meant was that it doesn't affect me at all, unless I reset, in which case I would just be thrilled to be able to do this.
We might not reap any benefits from it if we have already defeated them, but we sure as hell wont get any disadvantages from it.