Under this rationale, anybody who signs before the new tv deal would become a bargain because everybody's salary would go up. So if we didn't spend the money on Bradley and say, signed a free agent instead THAT player would also be a great deal! See where the logic gets faulty here?
Also, you're assuming 1) Bradley will improve over the course of the deal and 2) he will stay healthy over the course of the deal. I'm sure number 1 is a reasonably safe assumption though obviously he's not going to ever reach star levels. Health is where I get worried because his track record is not the greatest.
Finally, it's another question of who are you bidding against? Like with the Green contract there wasn't much of an impression teams were clamoring for this guy at 8 mil/year. There's also a line of thought that you can't have too many non-star players on these 8-10 million dollar deals because it just traps you in mediocrity. You try to load up on stars first, then find role players who take discounts or will accept the mid-level.
If Bradley is worth 8 million, what is Sullinger worth? Are we going to have an entire squad of guys making 8-10 million that will never make an all-star team?