I think you're mixing up a few things:
1) RPGs are rooted in wargaming (turn-based combat). However, this is not true for PnP RPGs. There is a bit of a difference between Homo Erectus and Homo Sapiens, even though both are part of the same evolutionary stage.
Yes. Wargames are Homo Erectus in this analogy. RPGs are Homo Sapiens. People pointing at a chimp and saying "Akshually this is real Homo Sapiens" would be the equivalent of people saying games without turn based combat can be RPGs.
2) I don't think that "RtwP games with combat based on Diablo" were the reason that your kind of perception ("tactical games are RPGs") was destroyed in mainstream. Especially considering the classics such as Icewind Dale or Baldur's Gate series (which are pretty much the epitome of "tactical games are RPGs").
BG and IWD had nothing to do with "tactical", they were extremely easy and cheesable, and could be played like an action game. In BG1 you could completely trivialize all the "RPG elements" and play it fully like an action game by abusing boots of speed which AI completely can't handle. All RtwP games have similar "exploits" rooted in their system. The whole reason RtwP exists, was because publishers had a hard on for Diablo, and it was impossible to convince them of funding an isometric game without similarly looking combat. Infinity Engine games' devs did the best they could under the circumstances, it was either making a bastardization of RPGs or not making anything, but
this doesn't change the fact it was a major step towards decline and destruction of what "RPG" meant.
3) If anything, Diablo is yet another example of "X with RPG elements". I could make an argument that if you have to define your game by using another genre as a prefix instead of using RPG as a defining category of it, then its place as an RPG is questionable to begin with.
It's the same as using RPG's as a prefix for Hack'n'slash games like Diablo. Hack'n'slash genre is deeply rooted in RPGs character management, it's basically RPG-lite with action oriented combat. It's impossible to talk about what Hack'n'slash is and what's its history, without referencing RPGs, despite it being a separate genre. Just like you can't talk about RPGs, without wargaming. The only difference is, thanks to H'n'S massive populartiy, it circled back and started influencing the RPG genre too, into adopting less tactical and more action oriented combat, blurring the lines between the genres in mainstream perception, while Wargaming stayed firmly separate.
Is it though? Isn't it a tactical game with RPG elements? Which then gave birth to a completely different genre (RPGs, instead of wargames/tactical games with RPG elements)?
How something could be "tactical game with RPG elements", before RPG genre existed? What you call "tactical game with RPG elements" is just RPG. When people took some RPG elements from it, and abandoned the core of the genre (turn based combat), there should be a new subgenre name created for it, but instead they just still called it RPG, which is the root of our current problem. Later it happened more times, with different devs removing more and more core elements of the genre while still calling it RPG, until the label lost any meaning. You can pick any iteration of the term, any game that has any "RPG elements", and call it "RPG" if you want, and it's equally valid. Except for the original iteration, what you call "tactical game with RPG elements" - it's the only one that is more valid, because it was first.
I would say that there is a need to define when something is a "tactical game with RPG elements" and when it's an "RPG with tactics".
Both are RPG. As long as it has stat based turn based combat, proper character management and narrative element to provide context and make you care about combat, it's RPG. You can add more or less of either element, you can add different elements, but as long as it has these core ones, it's just different flavours of RPG. Removing any of the core elements, should result in putting the game in different existing genre, or if other genres also don't fit well, creating a new subgenre or treating it as a fusion of genres (x with rpg elements).
So is Planescape: Torment then. A top RPG of the Codex for ages. What more needs to be said?
PS:T is closer to RPG than Disco, but it's still more of an adventure game/visual novel than proper RPG, and it should be in the "x with RPG elements" category. Just like majority of what people consider "RPG".
A questionable statement. Vampire: The Masquerade is undeniably an RPG, but it is not a tactical game, even though it has combat.
Same as above, it's an action game with RPG elements.
Of course, putting it in this category, doesn't really seem accurate, just like saying Torment is adventure game with RPG elements doesn't seem accurate since these categories will also contain many games which are much further away from RPGs. But calling them just RPGs also isn't accurate. There is no perfect solution, but I'd rather stick to the original meaning of RPG, than go mad debating which one of the next 1000 iterations can still be called RPG, and when it finally stops being one.
