The D700 was one of the best if not the best Nikon camera of its day. It’s not become a useless camera in the last 10 years. It would be very interesting if any one would be able to pick out the images taken by a D810 and D700 if there was a set of 10 images posted at 1024px on a website. I don’t think many photographers would be able to (I wouldn’t be able to), and let’s be honest non photographers don’t care what camera took it (or the iso value and if it’s a little too noisy).
Whilst I entirely get your point and nothing has made the D700 a 'bad' camera, however the MP limitations
might have a significant relevance
depending on the type of photography it is used for.
For example, when my D500 had to go away for extended repair (thanks Nikon Service!) I used a D700 for my wildlife photography and the difference was phenominal, both in AF and final image quality.
Basically, when shooting wildlife (mainly small birds, ducks etc) most of my opportunities are at a distance from the subject and this can obviously affect image quality ... even with a 500mm and TC this can be challenging but with the D700 it was just not possible to meet the challenge in the same way that it was with the D500 (or D810) ... there was a clear loss of detail.
Now using it for a wedding, a portrait session or similar, it may well be that the output difference in cameras will be minimal or even unnoticeable but for some other types of photography it will be dramatic, it was for me.
I no longer have my D700, I have scratched my 'itch' in trying one but I would not go back, whilst engaging in my type of photography.
I could see an unacceptable difference between the two cameras but I hope the O/P doesn't find the same to be a problem for him, with his type of photography.