No, not everyone can throw 90, no matter how good your mechanics are or how conditioned. I think almost everone can eventually throw 75-80, but most people don't realise how hard 90 is until you stand in there and watch one. Genetics will only take you so far, but the thing is if you are at the age where 90 starts to get feasible for some (17-19 years old), then you probably know whether you have said Genes or not already, and as such know if your wasting your time trying to get faster.
Thats not to say that Genetics in anyway limit your pitching ability. I don't believe Genetics have any role on control, or whether you can develop a nasty curve or changeup.
I bet only about the 95% of all people have the 90mph ceiling built in, and the percentage for a Nolan Ryan caliber fastball is probably around 99.9%, with a tremendously small percentage of those actually fullfilling said ability.