The kit lens price of the current EF 24-105mm f/4L IS will be hard to beat. It's been as low as $500 recently.
Canon also sell a 70-300 for under $400. Doesn't stop people paying more for the 70-300L or the Tamron 70-300 Vi DC that is also more.
Agreed - choice is good. But I wonder...the Sigma 35/1.4 is generally considered to be optically better than the Canon 35/1.4L. But I wonder how the picture would change if the Canon 35L were available for $300 cheaper
than the Sigma, instead of the other way around? Sure, some people will pay a premium for optical quality (and I'm one of them). But many people will look at the specs of the lenses, and when they're the same, go with the cheaper option...especially if that cheaper option has a red ring around it.
Also, the rumor doesn't indicate which lens series this one would fall into (A, C, S), but so far only S lenses have weather sealing, and I don't see an standard zoom f/4 lens as a 'sports' lens, so that may become another differentiating factor.
I did say 'hard to beat' but certainly not impossible. In fact, I expect if Sigma really does come out with the rumored lens, it will be in the C series, not weather sealed, but priced very competitively (equal to or cheaper than the kit price for the 24-105L). Sigma may also be banking on the possibility that Canon will discontinue the 24-105L.