Isn't the new Tamron 70-200 f/2.8 almost as sharp as the beloved Canon version?
Not from what I've seen. The new 24-70 VC comes somewhat close to the Canon 24-70 II, but there's a bigger IQ gap between the 70-200/2.8 lenses. I'd bet that the new Tamron at 600mm won't be as sharp as the current 100-400 at 400mm, and since you mention a $3K lens you must be referring to the pink unicorn 100-400 II, which if it becomes reality, I'd bet that lens at 400mm cropped would beat the Tamron at 600mm easily. Then there's the issue of AF speed, something Tamron lenses aren't known for...
Many people buy the 70-300 non-L over the 70-200/4L because the former is 100mm longer, has IS, and is a bit cheaper. If they cared about IQ, they'd be better off cropping images from the shorter lens with much better IQ.
I think the Tamron 150-600mm will be reasonably popular, because it'll be a cheap 600mm lens. But there's no free lunch, and the price of this one will be IQ.
Time will tell. But no matter how "sharp" the new 100-400 is, you won't get more resolution from cropping an image shot at 400mm, as shot with the Tamron at 600mm.
That isn't true at all. Depending on how good the quality of the Tamron is, it could very likely perform poorly enough that its 600mm end that the Canon at 400mm is as good or better. The lens' resolving power is ultimately determined by aberrations and aperture. At the very least, the wider f/5.6 aperture of the EF 400mm lens would give it a diffraction limited resolution (~123lp/mm MTF50) edge over the f/6.3 aperture (~104lp/mm MTF50) of the Tamron 600mm. At the very least, your losing about 15-16% due to diffraction with the 600mm lens. There are also going to be compromises in order to support the zoom range. For this lens to fit into it's cost bracket (i.e. reasonably cheaper than the Canon), you have to figure Tamron is cutting even more corners, so the optical performance of the lens is not going to be absolute top notch, which means your going to have some optical aberrations on top of the diffraction, too (i.e. it's best performing aperture, it's actual DLA, is likely going to be a stop beyond maximum, f/9.)
So, I wouldn't be quite so certain that a top notch L-series 100-400mm replacement that uses the latest Canon optical tech like antireflection nanocoating on internal elements, maybe a fluorite element or two, and Canon's new high grade manufacturing process couldn't hold it's own against a mid-range 150-600mm superzoom from Tamron. It is still certainly possible the Tamron at 600mm outperforms the Canon 100-400 at 400mm, and it might even be able to edge out a 100-400 II if/when it comes out...but I wouldn't suspect the margin to be more than very slim...not with a 150-600mm zoom lens...too many necessary compromises.
There's no way the Tamron would be that bad. If it were, nobody would buy it, and I doubt Tamron would build it. This isn't the 1990's anymore. And from what I've seen, the Tamron 70-200 f/2.8 definitely is sharper than the difference in the price, compared to the Canon 70-200 f/2.8. 75% as sharp? More like 90%. 90% of the sharpness for 75% (or less) of the price.
What are you basing those numbers on? Gut feeling? It would be nice to have some actual numbers or comparable MTF charts to back up those claims... I know the Tamron 70-200/2.8 is a good lens, but saying there is only a 10% margin in difference between a $1400 off-brand lens and a $2200 brand-optimized lens just needs more than a gut feeling for basis.