Dear Steve and WeatherCat shutterbugs,
Sorry here is my belated reply.
The D200 compared to the D750 is like comparing a Mac Quadra to your current Mac. *Everything* is better!
Wow! I can see you are extremely pleased!
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.
Our experiences seem to mesh on this lens range. Indeed it makes sense to get the best possible quality lens you can get for this range - it will get the vast majority of use.
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.
Okay, I see your point. For vacations a longer telephoto has a place.
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.
I've never been a big fan of wide vistas but perhaps that was simply because I never had the tools for the job. Okay I see your point.
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.
Actually this is what I was expecting from you initially. I don't take a lot of photos like this, but I'm still not traveling much either. So this too may change!
Thanks for sharing your insights!
Cheers, Edouard