Theorycrafting is fine from feedback, but you need frequent tests to see how things work and its here where FD fail. Too often FDs ideas of fun are totally disconnected from what players think are fun. By the time the new feature drops its too far along to correct its flaws and you wind up with multicrew, Powerplay and engineering v1. How much time would be saved if FD play tested these features earlier?
For something like combat balance where you can just give people infinite money and materials, a does-everything engineer, and let them have at it, sure, that could work, depending on what specific aspects of combat balance were being tested.
For something like engineering ... well, v1 certainly didn't work out very well, but a lot of 3.0 engineering was criticised heavily in both theorycrafting and in the Beta because some of the bits which made it work only applied after it had been in place a little while. (e.g. How many people still complain about not being able to roll G5 modules any more now that they have mass stocks of G1-G3 materials anyway?) Other than the occasional complaint about material gathering - and Frontier could make that arbitrarily quick and some people would still complain - it's basically been accepted now the shock of "things have changed" has worn off.
For BGS or Powerplay, the length of time needed to reasonably test a change would be immense, and even a months-long Open Beta would be way short of real player numbers and activity levels. Obvious bugs might be picked up, but design and balancing issues could easily be missed.
For multicrew, well, there's nothing wrong as such with what the gunner or SLF pilot roles can do - it's mainly that there's lots of things they can't do, and the implementation is still flaky. That's not something early access to it - while the implementation would have been even flakier - would have fixed. And a lot of the things they can't do, they can't do because it's hard or has tricky implications (SRV access, for example), rather than because they don't think people would enjoy SRV multicrew.