I'm not entirely sure about all of that - as a rule, you have to pitch full as a bowler to get the ball to swing unless you're very tall. Hardly anyone can swing the ball from "back of a length". Andy Caddick is the only one who comes to mind that did that particularly well.
I agree with some of what you say, but I also think a huge factor in the lack of 90mph+ swing bowlers is the fact that so many put such effort in to get their pace up that they lose their seam-position. And a poor seam position = no swing.
In any case, it's not like 90mph bowlers are two-a-penny. But there's plenty of examples of bowlers of that pace (or almost certainly of that pace) who could and did swing the ball - Ray Lindwall, Fred Trueman, Jeff Thomson, Malcolm Marshall, Waqar Younis, Allan Donald, Shoaib Akhtar, Brett Lee, Simon Jones. And these are just a few examples - there are many more you could go into depth about.
It's physics, mate. Yes those blokes swung the ball but of the ones I've seen, the swing is usually quite late and it was the combination of a reasonable amount of swing + sheer pace that did for the batsmen they were bowling to. In terms of lateral movement, there's no physical way someone bowling 90+mph is going to swing the ball as much as someone bowling much slower.
People have tried many times to explain swing in terms of Magnus Effect which is wrong because it really only applies to drift in spin bowling; pace bowlers don't put enough back-spin on the ball for it to take effect. It actually depends on whether you're talking about conventional swing or reverse swing exactly what phenomenon comes into play. It's pretty difficult to find any recent work on this but in terms of conventional swing, it's been found years ago the optimum speed for gaining maximum swing is only about 80km/h. Quicker than that and the effect is gradually degraded proportionally with the speed (see Nature 303, 787-788 "Factors affecting cricket ball swing", Mehta et al). It's why quick bowlers who rely on swing are usually less effective with the new ball; the two W's were always considered better with the old ball and that's explained by....
Reverse swing. Different fluid flows occur with an old ball on either side and this is where Bernoulli's Principle comes into play. Without going into too much detail, on a cricket ball which has a really rough side, essentially the difference in flows across both sides creates a pressure differential and subsequent change in air-speeds on either side, pushing it in the direction of the side with smoother flow (the shiny side). The faster you bowl, the later the swing. And, as I said above, the faster you bowl with convention swing, the less swing you'll get. Not none, less.
I haven't read much past this post, so I don't know if anyone mentioned this, but I think that Kallis might be the man for this job. On the evidence of the present series in England, if not anything before it. Kallis seems to be putting in the hard yards in hope of success. If he continues to bowl with such focus and drive, he could be the key for South Africa with the ball, as strange as that sounds.
The only issue I have with that is workload. He won't be able to bowl for long periods into the Doctor and then bat and play big innings'. Considering he's a swing bowler, he's a decent candidate, though.
