So how is that different from your D200?
So what sort of photos were you hoping to catch with your 150-600mm and the new 15-30mm lenses?
The D200 compared to the D750 is like comparing a Mac Quadra to your current Mac. *Everything* is better! More resolution, a full frame sensor instead of an crop sensor, 1/8000 top shutter speed vs 1/2000, hugely better focusing and exposure algorithms with many more sensors, and on and on. One huge one that I've experience is with the D200 you only used ISO 800 and higher in dire situations, because they were going to be really noisy. Walking the beach in Florida in February, I took a photo of my brother and our wives at ISO 51,000 hand held. It was so dark I could barely focus on them, but the photo came out reasonably well. With some post processing, I could most likely take out most of the noise. All that, and it is lighter than the D200, and cost only a little more. Ten years is a loooonnnnggg time in cameras these days.
On my D200, I used the Nikkor 18-200mm f/4-5.6 the vast majority of the time. It was one of the better midrange lenses offered at the time. With the crop frame sensor, the image coverage was the equivalent to 27-300 (1.5X crop correction.) With the D750's improved quality, it would show the shortcomings in the full-frame 24-300mm lens, so I opted for the much better quality 24-120mm fixed aperture f/4 lens. Again, this will fill a vast majority of my needs. Except when it doesn't. On the D200, I'd switch to my 300mm f/4 (450mm crop equivalent) if I needed longer reach for critters or something I couldn't get physically closer to. That's what the 150-600mm will be used for, too, in spades. In Florida, I took a photo of an alligator's mouth, not just the alligator. On our western trip, I could have zoomed way in on the grizzly we saw, or the bison and elk. Just a lot more options.
On the long end, there were always scenic views I couldn't get in one frame, and couldn't back away further, especially out west. We were at Crater lake, and I don't have a single photo of the entire lake. Just several with 1/2 or 2/3 of the lake. Having a 9mm wider lens will make those possible. Interior shots, too, such as churches, museums, and such. Anyplace where I can't get everything in frame that I want. Some of this could be done with multiple photos and stitching, but I'd rather do it in one shot.