I am glad we seem to agree that this is just about a larger minority of high budget SLR users like you changing to 35mm DSLR format, rather than an absurd prediction that the smaller DSLR formats wil cease to dominate overall digital SLR sales.
A few facts though:
- the price difference between the 20D and 5D is about US$1800, well over the $1000 in your $500 vs $1500 comparison. Sensor price difference seems to be the main factor in this US$1800 difference.
- The format price difference goes up if you wish to get the much talked about better high speed performance in telephoto shots, because to use the same ISO with a larger format needs a longer focal length but the same minimum aperture ratio. Compare a 200/2.8 to a 300/2.8 for price (and weight), for example. What is more, the same minimum aperture is more often not even available for the longer focal length: where Nikon DX can uses 200/2, 300/2.8 or 400/2.8, the equal f-stop 35mm same FOV counterparts of 300/2, 450/2.8 and 600/2.8 do not exist. If instead you use the same focal length with the larger sensor and crop, all the image quality advantages of the larger sensor are cropped away.
Presumably your prediction about the smaller DSLR formats (please, can we avoid the inaccurate and derogatory term "cropped format"?) needing "a full range of high quality EF-S lenses" only refers to Canon's EF-S mount DSLRs; the survivial of DX format, Four Thirds, etc. do not rely on EF-S lenses!
Even then, you have not offered the slightest evidence that the price penalty of the larger format will ever be low enough for the majority of SLR buyers: US$1800 is far, far too much: it is more than the gap between entry level and top of the line in 35mm film SLRs! Looking at film SLR prices, every few hundred dollars leads to a substantial reduction in sales volume.
Finally, the current range of lenses for EF-S mount DSLRs is probably adequate for the great majority of SLR users. What lenses do you think are missing for EF-S mount that are relevant to more than a small fraction of SLR users? Hint: mainstream SLR lens usage these days is dominated by "non-pro" zooms slower than f/2.8, along with primes at the wide and telephoto extremes of f/4 or slower, and maybe macro lenses. Everything else is low volume.