It's opinion based, because "What is an RPG?" has always been opinion based. Other genres are well defined, a racing game? Everybody knows what a race is. A FPS? You play in first person and shoot.
RPG has never been so clearly defined. From its first days in the 70s, RPG players always land somewhere on the axis between "I care about the game mechanics and optimizing my character, I enjoy the game the most, when I play optimally and do what is best for me and my party" and "I care about playing my character, I enjoy embodying him/her and experiencing stories and adventures in his/her skin". Another axis is combat encounters vs dialogue and storytelling.
The thing is, RPGs allow players of both camps and even wherever you sit in between to enjoy the game and as such despite these potentially radical differences in mindset there never developed a different terminology, after all whether you were powergamer or storyteller, you both played for example D&D.
This continues to this day and even many Computer RPGs can be affected by it. Take for example Witcher 3, you can be equally likely to enjoy it because of the combat and character levelling, the story, dialogue and decisions or any combination of the two.
An RPG is such a broad and versatile medium, that its definition will always remain nebulous and making it impossible to answer to the question "What is an RPG?", for one person Diablo may be an RPG, because it allows to level and equip your character, delve into game mechanics etc., another may laugh at the threadbare story and non-existant decision making and deny it is an RPG.
For this person, Age of Decadence may be the perfect example of an RPG, a game, which you can play without ever entering a fight, a thought abhorrent to the first player.
What makes a game an RPG is different for every person, as such everybody has to form their own opinion about the matter.