The Sigma 50-150mm f/2.8 is an APS-C lens, and thus a 70-200mm equivalent. For crops, it would be a good alternative to a 70-200mm.
It does appear that Canon builds their line-up around the 70-200mm zoom. Alas, I don't build my personal kit around it. It's too heavy, and my 135mm gets the job done on the long end. Plus, the weight and size of the 70-200mm takes the fun out of it (for me). Honestly, I would rather just go all the way and rent a 200mm f/2 if I'm gonna break my wrist for an ultra-flat portrait with a creamy background.
All we are saying is give different focal lengths a chance. :-)