Have to admit my initial thought was that the Saints got fleeced, but a Saints poster made a similar case to you - average ruckmen who is also average when resting forward.
I think it's definitely a case of wait and see what they've got at the end of the trade period. Currently they've got two players taller than 195cm with AFL experience on their list. And Rhys Stanley is one of those and I thought they were grooming him into a defender. It's pretty safe to assume that they're not sitting on their hands for the rest of the trade period. Although they did try everything to get a key defender last trade period and ended up with nothing.
All in all, I think Savage + pick 17 seems fairly reasonable for McEvoy, whom I also think is a bit overrated (he was significantly better at age 22 than at age 24, which is pretty alarming). It's also reasonable compensation for Franklin, which is ultimately, pretty much what it is from Hawthorn's perspective. But whether it was actually a smart deal for St Kilda depends on what else they can pick up over the trade period. Because right now, they look either very old, completely unproven, or woefully undersized in their key positions at both ends and the ruck.