microcontrollers are basically micro-computers
Not so, they are designed for fast I/O applications, not number crunching.

the bigger version won't be as much responsive, but will have enough power to calculate what is needed to compensate a system error.
You can get an ARM that run as fast as a Cygnal

A mixt system containing a computer and a controller based system is also interesting if you wan't to have top performance.You can or example use you 8051 as a light PID controller to have faster response
Another possibility is a multiprocessor design which, with small uCs may run as fast as multiprocessing with a 32bitter

Last point, for the computer solution is taht you can easily find hardware, with drivers or programing interface to shorten significally dev time.
The whole probleme is "How much does your time cost"
maybe, maybe not. I have spent 2 weeks to save $0.10, but then, of course, 500.000 units were to be produced. Considering the cost of development is irrelevant if not related to the production volume.

Erik