> Given they still offer 3.5, three years is probably a more fair depreciation schedule.

But the usage should drop considerably as soon as next model is released. Many startups down the line are existing in hope of better model. Many others can switch to a better/cheaper model quite easily. I'd be very surprised if the usage of 3.5 is anywhere near what it was before release of the next generation, even given all the growth. New users just use the new models