I got you covered, somewhat.
My Playing Roles series is basically an academic thesis asking the question as to what is a RPG, to the point where I plan on taking all of my articles and writing a book on it.
I was sure Telegard was turn-based. Like many games of the time, the turns had a timer (if you walked away from Questron and left it running whole you stood in a town, your character would eventually starve to death), but I thought it was turn-based, which to me would make it very different from Diablo.
But it's been a really long time since I played Telegard.
To me, it's easy to draw a line (as you do) from the tabletop RPGs of the 1970s to the earliest CRPGs (which were, as you say, often just adaptations of tabletop rulesets). The leap to JRPGs is harder to make.
Especially if you tell the story in chronological order. But you didn't do that. You talked about the very earliest CRPGs as dungeon crawlers (which they were), but once you got to Wizardry and Akalabeth you jumped to Japan in 1984, which might give someone the impression that western RPGs were just dungeon crawlers until Japan introduced the concept of story and characters. You even go so far as to say that the western developers of the 1990s were the first to introduce nuanced plot lines in western RPGs, when there are at least 4 Ultima games that did it first (U4-U7).
But earlier games like Ultima I and II had stories, and they'd begun to flesh out the world. Many western RPGs from this period focused on overland exploration and introduced detailed NPCs (you do get to these later, but I think you rush past the 1980s in the west and get right into the decline of the 1990s when CRPGs then had to compete with consoles again, and the JRPGs were first entering the western market).
Yes, the western RPGs had stagnated somewhat in the absence of any meaningful competition, but we're talking about a 10+ year period. Lots of games got made in those 10 years.
You also point to the Ultima series as an exception, describing its use of story despite being a western game, but the exception doesn't prove the rule. Exceptions disprove rules.
Questron had a story. Bard's Tale had a story. Might & Magic had a story. They weren't as tightly woven as the stories out of Japan, because the games were doing something other than telling a story. They were setting you free to explore the world. The story was just one thing you could do.
JRPGs get lumped together with RPGs because of the name, but they arose from a history of storytelling, not roleplaying.
The ultimate truth to that answer is a nebulous one, it doesn't have a clear definition because it crosses over into a lot of genres and design philosophies. Even Chainmail and first edition DnD is completely different to what constitutes an RPG today, to the point where tabletop games from the 1980s would be seen as a different genre of role-playing game. With video games, the classification is completely different, but should be done by describing the type of game. Games, and players, have changed as tastes and ideals evolved. It's the way of things and ultimately makes the true definition of an RPG meaningless to define outside of mechanics you expect when playing it.
Like, a tactical RPG vs a action-RPG, or a rogue-like, or a dungeon crawler, or a MMO. That is how you form that definition, in my estimation at least.
None of that has anything to do with roleplaying, supposedly the core feature of the genre.