Didn't even think that the grass sword would end up being the cause of the loss of his arm. It does have to happen, it's just a matter of when now. It could even be what needs to be sacrificed to get to the
Still need an item of immense power to open the gate since there is no more Enchiridion.
Tonight's episode though was really good with the build up to the end. Wasn't even thinking much about a massive reveal, but there it was. Now April cannot come soon enough. Our little Finn is growing up.
Edit: Pretty much every alternate version of Finn we have seen so far, including those mentioned show off the mechanical arm. It's inevitable because of the consistency of the visions and past/alternate lives. It's even hit upon in the comics, which aren't canon, but I think are close enough to count for me. Plus that grass blade ain't coming off any time soon. It's cursed.