I believe that it is possible to make a 600F5.6 lens for $2000. What I don't believe is that there would be a good market for it... Those who must have the very best will go for the $12000 Canon, and those on a budget will go for the $1100 Tamron zoom. This really pinches the market... but there is no way I would place bets on this... Who knows what Tamron and Sigma have up their sleeves....
The argument isn't that it's not possible...its that if someone did it, for that price, would it be marketable?
I'd be willing to bet that Tamron has already cut some corners with the 150-600. We know Canon does this with their zoom lenses...instead of 600mm, it would be something like 149mm-590mm. That immediately shaves a bit off the necessary front element size. The filter thread size is 95mm, which right off the bat indicates that the front element can't actually be the 95.24mm it would have to be in order to support true f/6.3 at exactly 600mm. That's a reduction in objective element size of about 5% right off the bat. That shaves down the necessary size for all the entrance pupil elements as well.
Prime lenses are generally designed to more exacting specifications. You don't have to worry about making tradeoffs to support a zoom range...you optimize the lens design exactly for the focal length your building. That's the entire point of a prime...to extract the maximum quality out of a given focal length. So, at least assuming you want a 600mm f/5.6 lens and not a 590mm f/6 lens, that immediately cuts out any easy reductions in element size that might save you a few percent here and there to reduce the cost of the lens. Then you get into how larger optics are harder to correct, etc. etc. and the costs just start rising from there.
I guess it would really boil down to what level of IQ you think your customers would find acceptable. Perhaps a 600mm f/5.6 that had worse CA than the 150-600 f/5-6.3 might indeed sell, to the people who insist on having a prime. Personally I think the customers in this bracket would go with the best option for the best price...that's kind of what it's all about...so I don't think a 600mm f/5.6 with worse quality than the Tamron 150-600 would really have a market.
Also, keep in mind, Canon's prime lens with a 107mm objective element costs $6800...the EF 300mm f/2.8 L II. Granted, that pushes lens design to the upper extreme of high quality, and employs the most advanced optical technologies known to the DSLR market...but still, the vast majority of that cost is the glass.