Related to ongoing contracts that are late and run on AIX 5.3 perhaps
IBM tenders on a contract, uses latest and greatest, fails to deliver a 2 year project a bit late and all that lovely tech is out of date.
That all said it would be somewhat pushy to assume that everybody updates OS every time you release it. If the curent one works fine and the kit is due to be replaced/upgraded in a 5 year cycle; let alone that they sync up with IBM's release cycles, then unless it's LPAR'd up onto a newer box it will just sit were it is, working, doing it's job, working fine. Also upgrading any OS version, nomatter how smoth will always have gotcha's (thining MQ queue storage few years back myself) with the software that runs upon them.
Another view of all this could be that IBM is very good at playing poker and only when enough call there bluff do they play the cards they were prepared to play from the start. But at least gain some panic early upgrades and most people who get a AIX box will in general just apply PTF's (permenant tempory fix's(sic)) were needed and like everybody know's, if it works and works well without issue then who are you to introduce issues.
Bottom line those who did not but the latest kit on day one of its release will still get tehre 5 years support and accountants will sleep at night, curse you IBM, curse you.