The 500mm+ primes cost far more that a comparison with them is pretty much irrelevant.
The lens is 600mm so a comparison with 400mm+1.4TC can be relevant, but they are $800-$1,100 more expensive, start at f/8 and some of those options don't have any zoom. So for many people that is already a different market and actually irrelevant too.
Re whoever mentioned this is "just" $120 more than the 200-500mm. I'm pretty sure Tamron advertises this is a replacement for that lens, so I guess they couldn't or didn't "want" to make it astronomically more expensive (OTOH there's Nikon...).
The USD, VC, 7 more elements, coatings, etc. may have been far more expensive to make in the past, but just like a current $500 computer is far better than a $4,000 computer 20 years ago (purposely giving an extreme example), it's possible (even likely) that all those things cost them far less now. Maybe they have nearly only production cost for them. At least with the USD and VC they have them (and they are excellent) on the $450 70-300mm lens.
Re the post about Tamron not finding a "magical" way to make an excellent lens so "cheap"... just because there are better lenses doesn't mean a lens is not good, or even excellent. Just because the 70-300mm isn't as good as a 300mm prime or the 70-300mm L doesn't mean it's not good. It is very good IMO and possibly the best value in that range.
I don't think anyone knows where the 150-600mm is made yet. It will probably be written on the lens. My 70-300mm is made in China. The better Canon lenses are made in Japan. This alone can make a huge difference in price, seperate from quality.
Plus, no one knows how Tamron really works in terms of what they do and what they have to charge. A random example, it's possible that two people come up with a similar, worse, or better lens design than four people, but four people cost double. This is just a random example but no one really knows what the situation is for any of those companies for them to sell any lens for the price they choose.
So, I expect, or at least hope, that this new Tamron is at least as good as their 70-300mm, which is sharp and not too far from the Canon L lens. For anyone who was hoping it would be $500 more for better quality, who knows if making it $500 more expensive could really make it better quality optically. It might take far more than that to make it better in a significant way. IMO it only matters how the lens really is, how much it actually costs and how it compares with lenses that are worth comparing it with. When it is available here I will definitely go and try it.