AFAIK Tamron/Sigma do sell tons of 24-whatevers and 70-200, just like Canon does. You are correct that there is no point in Tamron/Sigma offering an 18-55.
Yes indeed; that's why I said "focus on price and filling in...". They sell the 24-whatevers and 70-200s for cheaper than Canon's, for people who can't afford those or choose not to.
I don't know if my previous post was unclear or people are just trying to nitpick.