- Unreliability of USB3.0 and sataIII for real time control and feedback
I don't think the reliability of USB or SATA is the issue. It's the UNreliability of Windows that is the biggest problem!
Some other points should be made here as well.
The older closed-loop control was created before the n86 chips reached their current level of speed. They relied totally on dedicated hard-wired logic chips to do the motion control for servos: there was no other way to get the control for linear servo motors.
Mach is, to some degree, based on the concept of a stepper motor. You do not (really) need closed loop control with steppers: either they have the power to drive the axes, or they don't. Feedback will not change this.
Working forward from the idea of steppers, some drive units for linear servo motors now emulate a stepper. You feed them 'stepper pulses' and they convert that to DC (or AC) servo drive. If the system is working OK all is well; if the servo drive cannot cope it flags a fault and Mach3 stops. This is conceptually the same as a stepper system.
You are right in your statement that SmoothStepper has nothing to do with this. In this context it is just a logic interface, nothing more.
Now, going on to smooth changes in acceleration (3rd derivative) - that is more difficult. If it was really necessary then customer demand might lead to Mach including it. But normally you get this for free from the power amplifier: it cannot produce instant acceleration in practice, as there are peak power limits. So for most people there is no need for Mach to do this.
Cheers