I feel the core genre identity of RPG is a known thing and not as uncertain as you paint it. There's the iron-clad center-point with CRPGs and JRPGs. Then games that venture off from those identities into more action-y RPGs (a la The Witcher or Mass Effect). Or games that go more action-y but in a different way (Diablo-clones). There's games expanding out from the JRPG core like tactical RPGs (though there's an intersection with CRPGs somewhere there e.g. X-Com). And so on.

Sure, there will be games out there where people will ask "is this truly an RPG?" but that doesn't mean the genre itself is fuzzy and poorly grasped, even if it will be difficult to come across a satisfying definition.

The name itself is vague and a poor guide... but that's true across most gaming genres. People use "strategy" in shooters or RPGs or puzzle games, but we all know what a Real Time Strategy game is. Almost every game has "action" and a smaller but still nearly-every game has "adventure" to it, but action-adventure is another known quantity. I'm not sure there's any genre that is perfectly encapsulated within the name given to it, or one where there are not people questioning games at the fringes of that genre.