I have seen some people, IDPA Masters, who don't shoot USPSA at all. I see others that go on to USPSA and do both.
But there is no doubt about people improving while shooting IDPA in every aspect of gun handling. A lot of people....shocker....are not interested in buying a $2000 STI because that's what most guys have, and becoming a USPSA A Class or a GM. MOST PEOPLE who shoot IDPA want to get to a level where they are confident in their own pistol shooting in a variety of situations and don't really suck...to use your negative term. They can do that in IDPA...and they don't need to go beyond that. I will still put any experienced IDPA shooter up against a common criminal shooter any day. That is not to say there aren't people who don't continue to improve...there are. That happens for a variety of reasons, but the most common one is they simply don't have time to commit the number of hours needed to dry fire practice, nor the discipline to work on their speed and their grip and their aim. Other reasons are old age, injuries, and just not motivated enough.
If you want to work on becoming a GM or A class, more power to you...seriously, I hope you make it. But it is a mistake to think that everyone else needs to be there in order to defend themselves.
Cody