Wow, excellent post! I completely agree with you about everything.
I am a history buff myself and the LEGO castle and pirate themes of my childhood played a large role in my initial interest in history. I was also in a dark age and it was the Vikings theme that eventually brought me back to LEGO as an adult.
In regards to the Nexo Knights theme, I was originally apprehensive when I heard/saw it at first but I thought it would eventually grow on me. It hasn't. In fact, as time passes and more sets come out I find them less and less interesting... I don't even bother looking at the newly released sets at all anymore.
To me, Nexo Knights, Ninjago and Chima might as well be one big theme because all these themes seem to be just a group of good guys going up against a bunch of baddies with a bunch of crazy, over the top colorful vehicles. At least with Ninjago and Chima there was a few sets here and there that I would cherry pick because they'd fit in with my other castle/pirates stuff. I don't find this with Nexo Knights at all, I bought a few of the 'ultimate (insert name here)" sets but none of the bigger sets appealed to me at all.
My biggest fear is that Nexo Knight will just go on and on and on like Ninjago seems to be doing (didn't that theme start in 2011 or something?) which will make a classic castle theme most likely impossible. Instead, I'm just hoping to ride it out and maybe in a year or two a different, more historic or fantasy oriented castle theme will return.