Sub $1,000 full frame bodies will be coming sooner or later, at which point APS-C becomes increasingly less appealing for the kind of people you see around here. In the long term I think APS-C will be relegated to the same sort of status your average P&S has right now.
Hmm.... I don't really think so. Already the quality difference between APS-C and full frame is pretty marginal. In the vast middle where most pictures are taken and displayed it's impossible to see any difference. The bulk of research and development dollars industry-wide is going into improving smaller sensors for cell phones. Advancements at that level will benefit APS-C more than full frame.
Even if we see a sub $1,000 full frame in the future, most consumers will view that as too expensive and will opt for the APS-C camera at half the price or less. (The top four selling DSLRs on Amazon right now are all under $500).
But, maybe I'm misunderstanding your post. Perhaps you mean that enthusiasts will convert to full frame. That could be possible, but judging from the sales of the 70D and the pent-up demand for the 7DII, it appears that the market for higher end APS-C cameras will remain strong for the foreseeable future.
On the other hand, if you are suggesting that the niche that was formerly filled by point and shoot cameras will be filled by small DSLRs, such as the SL1, I'm inclined to agree.