Other than the very small sliver of higher-end users that prefer APS-C for what they do (see my prior comments on birders and sports guys), won't the folks more likely to buy higher end glass also be the folks likely to go FF? Ergo, is the 6D the beginning of the end for relatively high-end EF-S glass like the EF-S 17-55 F/2.8 IS or the EF-S 10-22?
That's a pretty hard question to answer, but I think the importance of the 6D would be that it will be the point for Canon's to show hands on its final stance for APS-C bodies/lenses.
With the introduction of the 6D, entry level FF under 2,000 is now a viable market. It will be interesting to see if Canon continues with manufacturing Rebel/XXD for the casual users who wants a DSLR body (which means the EF-S will continue to prosper) or whether they'll try to "shift" this market into the EOS M world (meaning goodbye EF-S, hello EF-M mounts).
My bet would be that EF-M would emerge as the winner, since the EOS-M already offers mount adapters for the bigger lenses (so those with investment in EF-S lenses could still use them) and includes the same APS-C sensors.
Now, Canon seems confident enough they can produce EF-M lenses that are way smaller than their EF-S cousins but still offers the same quality (such as the EF-S 18-55 f3.5-5.6 IS II). Whether they can make a smaller version of the top EF-S lens, it still remains to be seen.
By the time they produce the EF-M 10-22, it'll probably be clear to everyone that the days of EF-S lenses is coming to a close.