The main difference between your suggestion and the older Nikons is optimal quality. As much as you may love the convenience and price of the Sigma zooms, they're hardly the pinnacle of optics
You obviously don´t know about the 120-300 OS. Read the reviews and check it out first.
The resolution of this thing is about on par with many tele primes. Better than any other Sigma telezoom has been.
And your presumption that I love the "convenience" of zooms is absolutely wrong. There is nothing convenient about zooms. I would much prefer a prime, but I will not shell out 6k for a 300 2.8 prime if a fast-focusing, stabilized, really sharp zoom is available at 2k. Apart from this one, I do not own any other zooms and I doubt I ever will (a series of 8/24/85/300+ primes would be enough for the rest of my life).