I believe P4 was designed and is very useful for rookie, poorly funded, and/or struggling teams. It isn't like you purchase a ROTS and then compete. You still have to do work in terms of getting it working and what not. You still have to do some work to see what COTS parts work best for your design.
As those rookie/poorly funded/struggling/all of the above teams start getting more and more experience I think that they'll naturally advance to fabricating their own parts, maybe using the COTS parts as a reference or and idea but designing and fabricating something unique to their team. They'll realize that these COTS parts are designed for more general use, and they'll want to create something specific to the game. I also think this will naturally happen because many will agree that designing and building something by yourself is much more satisfying and fun than buying a kit and putting it together
tl;dr P4 is for starting/struggling teams and those teams will naturally progress to fabrication from scratch as they get more experience.