I would tend to agree with the bold part of your post.
Anyone who is being honest can tell Diaz didn't want to be in the Octagon against GSP. He still had brief moments of success too. It's sad to contemplate what could have been. It was only a little over a year ago and it already feels like much longer with both of these great fighters retired.
I think we missed out on a great fight and we're missing out on many more now. But like you said Nick doesn't want to fight any more and he's more than earned a retirement.
Thats because he knew he had no chance of winning and only wanted a pay day. Don't kid yourself with this "what could have been" bullshit. Nick has lost every time he has stepped up to a elite WW and he always will