Let's not be too hasty with the presumptions mate.
SDM to Pentax is no failure, but the DC motor is an overhauled improvement.
There may have been limitations to improving SDM as it was, and so needed a change of tack.
In any case, SDM was bound to be a problem given the electronics (vs the screwdrive's mechanics) that are more prone to kicking the bucket. Whether there are higher failure rates with SDM as opposed to HSM or USM, who really knows, but each have their failings, whereas screwdriven lenses don't.
So is Pentax admitting SDM was a flop - nope, they've been flat out denying that to date. And I'm not convinced it is myself (well maybe a little...), on the overall scheme of things, even though I know there are many stories of SDM motor failures.