I agree, the forced rotation DA should be even more efficient than just larger throw non-forced rotation DA, it will never bog down. Relative to 8mm throw, a larger throw should always be more efficient at polishing and cutting.
I think the Flex is a great tool, but my hands are in the beginning stages of psoriatic arthritis, and the vibration on the 3401 was still doing a work on my hands after some time with it. I don't know if it was just the specific one I was trying out for the few days that I time with it, but I think I expected it to be a bit easier on the hands. I felt much more comfortable with my rotary, and if I have to spend upwards of $350, it's got to be easier on the hands than the rotary for me. But when it comes to the Rupes 21, I can't spend that much money on a tool that is going to bog in the curves and make me think about using a separate tool for that spot.
I couldn't throw away my money fast enough for a $200-$250 12mm throw near vibration free non-forced DA with the ergonomics of a compact rotary like the Flex PE14 with a slow start trigger and torque control. Something like that may even bog in the curves a little bit, but it will make me put down my PC7424 for life and make me think hard of ever going to the rotary again, and a little bogging in the curves is fine if I'm not spending close to $400 for it.
Valid point, I may be very naive to think that it's that much easier to go larger than 8mm cheaply for manufacturers.
