You are correct about the pass rate, and the limited number of people qualified. Although I don't think it's that it's a hard test, I think it's the later, lack of people taking it who are qualified.
The 510/570 crowd for the most part are good at what they do, but even with them, most are limited to specific aspects. Within the API world is a whole subculture of myriad specialities. A guy working LNG is not necessarily qualified to work a refinery, or vise versa, or off shore, or the many other things API has their hands in and even in those, there are subgroups. Just the RP2X offshore tky UT cert has been pigeon holed to a realm of very finite expertise, then there are the host of RP's that require specific experience to work to. Passing the 510 and 570 is no problem comparatively, the 653 is only a bit more difficult. Where the rubber meets the road are the gurus who work the speciality work. You have to know it even exist to get into it, and at that someone typically has to bring you in. I can equal and exceed the rates for 510/570 without ever hitting a lick for either cert in those specialities that I personally work to. Sooner than later the number of 510 570's will exceed the number of CWI's. With the work load they will have to if the work is going to get done.
With current projected work, I could never leave the LNG/cryo shipping market and retire from it if I wished. There just aren't that many people out there with cryo experience of any kind. Thats not even mentioning the upcoming LHG currently being pushed and researched in some Asian and Euro countries. Since the majority of API's work is focused on higher temp services, I don't see it changing anytime soon since they can't even get the people for it much less the cryo. The energy has to come from somewhere, and the bottom line is there is a finite amount of it available in the world. Should I live a normal life span, I firmly believe I'll see it (LHG and fusion plants). Either way, just LNG will keep me busy for the forseable future.
My two cents worth