Asics change a lot between generations. In year 2020 all S3's will be sitting on shelves.
No one will still be using them in 5 years, it's just that simple.
Why ?
Profitable will be same.
Than i will buy s5.
Even if you buy a S5 chances of it being used in 2020 is slim. A miner's lifetime is no where near 5 years. The advancements make this impossible.
They are able to do more power for less electricity each generation. It does not matter what miner you buy in 5 years it's to old.