Even with Outposts you're not "buying" the rack (Amazon still owns it and takes it away at the end of the term), you're just committing to keeping it long enough for them to make money on it. The choice of paying upfront or paying monthly is roughly the same money management strategy available with any AWS service (e.g. EC2 Reserved Instances), and likely is more about lining up with capex/opex decisions than actual cost savings.