Agree with both statements.
I had the 70-300 Tammy and it was a lot better than the Canon in handling (non-rotating FE, manual focus override, better build), was about 20% cheaper, and about the same in IQ.
But considering the fact that Canon is around 1.1K with discounts and street prices, I think the market will be tough for a third party lens, especially given the L zoom's reputation.
There's a $600-$700 gap between the Tamron 70-300 and Canon 70-300L.
And Sigma don't yet have a good offering for that focal length range ... their 70-300 zooms are very very ordinary.