fike,
1. This prediction has been around at least since the 1Ds was announced almost seven years ago, and persists despite no sign of APS-C slipping from its overwhelming dominance of DLSR sales, including recent APS-C models at well above entry level, the 50D, D300, A700 and K20D, which together greatly outsell all larger format DSLR's combined, as far as I can tell.
2. Do you think that everyone wanting more than entry level SLR kit needs or wants significantly more than 15MP? If not, why would it matter if APS-C could not go much beyond that? (Not that I believe that APS-C is so close to its sensor resolution limit; lens resolution might be the limit that comes first.)
3. APS-C is not the same as EF-S, so it misses the point to look only at the limitations of Canon's EF-S offerings. Canon has in many ways held the level of its EF-S offerings a bit lower than other DLSR makers. The Nikon DX and Pentax DA lens systems offer a number of lenses at well above the "entry level", and I would include Nikon's latest lens, the 35/1.8 DX, in that list. (Not to mention FourThirds lenses, another system that shows no trend towards becoming "entry level only".) Actually, even Canon has more than those two EF-S lenses of "better than entry level" quality: add the 17-85 and the 60/2.8 EF-S macro lens for example.
Further, APS-C DSLR's can happily use 35mm format lenses for most telephoto focal lengths, so EF-S, DX etc. do not need to offer good lenses at all focal lengths. In particular the 17-85 EF-S and 17-70 DX reach long enough to match up nicely with 35mm format 70-200 and 70-300 telephoto zooms.
4. As someone else has said, size and weight advantages will always be a reason for some serious photographers to choose APS-C (or FourThirds) gear, even those who also use 35mm format or medium format gear for other purposes. Our common interest in hiking photography is one example!