You can make or agree with hyperbolic comments like "Well, the days of Canon/Nikon taking their own sweet time to update lenses is likely drawing to a close." or you can look at the ample evidence that it is not. Don't forget for one second Canon knows to the exact number the market for the 24mm f1.4 premium lens at the price point they are prepared to sell them at. Nowadays it seems the one who shouts loudest or wishes the most gets the most followers, that doesn't make what they are shouting accurate.
I think you are neglecting one important point.
Canon has a multilayer interest in producing lenses. First of all they produce a revenue on their own, otherwise they wouldn't design, manufacture and sell them. Differently from kit lenses and consumer cameras, the target market of expensive lenses is more opinionated and sensitive to quality. The moment you offer them better quality at a lower price, you make them interested. More and more as time goes by and both Sigma and Tamron become established as quality manufacturers.
Second, many people are with Canon because of the lenses and their reputation to be the best. The moment this changes, a big reason for having a Canon system is gone - especially if you take into account that other manufacturers offer better sensors too. So the availability of quality lenses and cameras in different mounts is eventually going to affect camera sales too. Case in point, the latest market share data for Japan showed that Canon is still the market leader but did lose some share. Sigma btw has a quite considerable share of the lens market.
So as someone stated before, Canon executives are not likely to start pulling their hair already, but at the same time I do agree that the Canon/Nikon duopoly is over and that both companies should really rethink their strategy.
Firstly, that was two points, not one.
To address your first, if Canon already make a premium 24/35/50 it becomes a consumer choice, but the high end appeal for many Canon system owners is the unmatched diversity in the EF lens range, things like the 17 and 24 TS-E, the 65 MP-E etc. Now what would attract more people to Canon, yet another 24mm option or a kick ass 45 TS-E? My opinion is the 45 TS-E, we already have a choice of 24's so nobody is going to leave because there isn't one, but people might come for the new lenses that offer ever greater diversity, using that reasoning it makes more sense for Canon to ignore Sigma's latest offerings, until they want to break the communication protocols again and send this second generation of Sigma EF worshipers into the depths of despair the first generation did, and concentrate on completely different lens designs like TS-E's, small medium speed primes with IS, video orientated AF etc.
As for your second, what is the point of a lens without a body? The reputation is not for a lens, it is for the images the system can create, if they keep coming out with class leading bodies, like the 1Dx vs D4s, the 5D MkIII vs the D800 and the 6D vs the D600 (which has been banned from sale in China!) then minor differences in lens output will remain irrelevant. Things like the RT flash system will win far more new Canon converts than a few lppmm on a test chart of a new version of a lens.
But we digress, for Canon stills shooters the writing is on the wall, video and surveillance are the new darlings and progressively less and less will be spent on R&D for purely stills orientated equipment. Canon are making brand new groundbreaking world class lenses at comparative bargain prices, it is just that we are not the market for them. I have exactly the same feeling for stills orientated shooters as I had for the development of film cameras when I bought my two 1VHS's. The only recent key stills only orientated development was the RT flash system, and it is a winner against a sea of copies, clones and competing third party options.