Bear in mind that Apple sees the useful lifespan as 5 years max as shown by their OS support schedule. Your HD is not likely to last beyond that anyhow. PCs are easy to extend the life of as you can always swap components as needed. With Macs, its best to sell just before the warranty expires and buy new (or refurb) again. Rather than opt for a fusion drive, why not just buy an external TBolt or USB3 SSD and roll your own? You won't really notice a difference in CPUs, even in 5 years, and if you do you can always buy an i7 then (they should be cheap as they will be old) and swap it in when you replace the HD and upgrade the RAM.