Upload & Sell: On
Not sure about the possible interpretations of this, but my take is:
Since you own the 28-75, it is definitely not a crap lens. The Canon EF 27-70 II will likely not have IS, in which case there will never be any 24-70 IS. You dont see any need to upgrade at all?
The SP 28-75 and SP 70-300 are toy lenses and you expect the Canon 24-70 II to be far ahead in image quality and hope it will have IS?
OK. I can be a little more specific.
A: I never said it was crap. I own the 24-105L IS and most fast Canon primes from 24/1.4L to 200/1.8L. I recently sold my EF 24-70/2.8L because I didn't use it much, not because it has poor IQ. I might upgrade to a 24-70/2.8L IS; otherwise, I have no need to do so.
B: I never said anything about toys. I do expect the 24-70/2.8L IS to be far ahead in IQ.
The image quality, build quality, and AF performance of the SP 28-75/2.8 and SP 70-300/4-5.6 VC are not as good as the equivalent Canon L-series zoom lenses. This does not mean that the Tamrons are not better than other alternatives, nor that they're not worth having. It simply means that I would prefer to wait for the Canon 24-70/2.8L IS, because I'm confident that it'll be better in most, if not all, meaningful performance characteristics. The Canon will probably be heavier, and it will definitely be more expensive.
Again, based on my personal experience, the SP 70-300/4-5.6 VC is more-or-less on par with the Canon EF 70-300/4-5.6 IS USM. The Canon 70-200/4L IS and 70-200/2.8L IS I and II are both significantly better than the SP. This should be no surprise.
Both of the Tamron zooms that I currently own are on semi-permanent loan to my daughter. She takes many wonderful photos with them. If they had higher resale value, she wouldn't have them, and I could afford a new set of windshield wipers for my Yaris. I apologize if this last statement isn't sufficiently clear.
BTW, if the next version of the Canon EF 24/2.8L is a "Mark II", then it will not have IS. If it has IS, then it will be called the EF 24-70/2.8L IS, and it will be the "Mark I" version of this lens.