I've co-opted a definition concocted by... IGN, of all places. It seems to fit well. The distinction between an RPG and an action game lies in the level of requirement on time-restricted decisions and performance. Soulsborne, Zelda, Nier, Ys. None of these are RPGs because the foremost requirement in the gameplay is on player performance. The most associated design element of RPGs, storytelling - is not the focal point of design.
I think there's allot of people who would object to Nier in particular there because Yoko Taro is constantly blasting a message in your face but any way I look at the experience I can't see how it's fundamentally different from the old God of War games. Every element of the design has an analogous counterpart. They're both action games. One is just smarter (or trying to be, depending on who you ask).
I know I may be redundant on this point, but think of it this way. What do you do in a Soulsborne game? You explore areas, finding collectibles, and reacting and learning to threats in real-time as they occur. What do you do in Devil May Cry? God of War? This comprises the bulk of the experience. One is selling something different from the other. Soulsborne sells atmosphere, mood, mysterious lore, but the vast majority of the experience is in real-time learning, reaction, and mastery of situations presented to you. It's an action game. There are moments when the action stops in Nier, but there are cutscenes in Devil May Cry too and God of War. Materially, why would you ever describe Zelda, Nier, any of these other titles as anything other than action games?
There are some titles that overlap. Action and adventure do overlap allot. Breath of the Wild could be described as both, but that's because the core design elements of the genres comprise a plurality of the game experience. There may be action RPGs too, but at the very least I can say confidently there aren't as nearly as many as some people say. Like, Tales of games may be action RPGs since a good chunk of the gameplay revolves around time-restricted decisions (combat) but the games are so lenient on your performance of those decisions that I'm kinda iffy on them. I think it might be fair still to leave out the 'action' part because the threshold for success is so low that 'action' can't fairly be evaluated as a chief element of game design, especially when the storytelling and roleplaying so often are centerstage in those games (extended cutscenes, loads upon loads of skits).
