If you are only buying a single spare that is the case. Support contracts make sense when you would want several dozen spares.

if a support contract is 70% of the cost of something, then for every 2 of something a spare is cheaper.

For Aruba I think it was 1:1; a support contract was the same price as the device itself over 2 years or something.

Support contracts never cover a "site" they always cover individual devices, so your cost scales with the amount you have... so I'm not sure I follow what you're saying..

Unless you mean to imply that you're replacing units more than once every 2 years, in which case of course you're right... but then I might consider going with another vendor entirely because that is a pretty terrible reliability record.