'gradual hire' sounds great in theory, but it is going to be a pretty unattractive option for a lot of developers. One problem is the lack of certainty - why would I get 'gradually' hired somewhere if I have other options where I can get a permanent job right away? The bigger problem is the lack of benefits, which presumably includes health care. In the US, at least, this would make it pretty difficult - buying insurance on your own can be very expensive and going without it is risky/costly, so it would be a pretty big problem for a lot of people, especially those with health problems or a family.

I think this method would highly select candidates that didn't have better options. The reason it works in your first example is that the person being gradually hired is employed by another firm that offers them stability and benefits in the meantime.