Tamrons are junk. They're good starter lenses but that's about it.
Have you looked at the new Tamron 24-70? From the limited time I rented it and the images I'm seeing, it's quite good. Also from the reviews I've read, it's very good. IQ, not quite up to the 24-70 v2, sure, but still around as good or a bit more than the 24-70 v1. Build quality seems quite good, although again, not quite up to L standards. But it's $1300, not $2100-2300 or whatever the 24-70 v2 is going for right now.
Now, most of the rest of the Tamron glass, far as I know you're mostly correct, but if their 24-70 is the new direction they are going it, they're likely to become a serious off-brand competitor for people who can't quite afford L and don't want to pay the vastly more expensive new non-L primes with IS.