...and I noticed something kind of interesting.
The biggest peak SK ever had throughout its entire life was August 2013 with ~4000 Steam players playing.
I looked through the release history, and interestingly enough that was very soon after the Forge update came out.
i.e. the update that everyone hated at first.
Don't you think it's kind of interesting that the update that got the most backlash from the community happened to be the update where SK had its biggest peak?
Or is it just that it's a new update so inactive players came back to check it out then left again?
Yes, when there is a major release, a substantial number of players return, to check it out, whether or not they end up liking it.
With regard to the popularity of the release, my memory differs strongly from yours. Initially, players were quite positive about it: no more elevator fees, some free crafting, battle sprites, etc. The backlash came later, when the drop rate for Radiants was decreased. That was in the (northern hemisphere) autumn of 2013, I think? A few months later the rates were increased somewhat, but not as much as players wanted, and the damage was already done.