Buffalo fan here. It's a great AAV for you guys.
IMO.....Casey was always looking for a shorter term with this deal, which is why Adams traded him. Adams is big into "guys who want to be in Buffalo"....and if Casey was angling to take a 3 year deal and then cash in at the end, there's no way that Adams would have been amenable to that.
The term makes some sense to you guys as well. Allows you to bring Ritchie along slowly and then build him up to take over Casey's role on a cheap deal/ELC. Plus, the low AAV on the shorter team fits your cap structure well.