You don't always need masters. A full stack engineer will build your a rails app that might work perfectly for the whole life of your business. Only when your

You don't always need masters. A full stack engineer will build your a rails app that might work perfectly for the whole life of your business. Only when your business scales (a lot) that you need people that are more and more specialised. And trust me, the first backend engineer you hire to optimise the backend build by your full stack engineer will probably not be the backend engineer that will be able to scale and shard your database once you reach more and more scale. Nothing is black and white.