What if Mass Effect Andromeda had a silent protagonist?

Here is the problem I see with that. 

RPG has no meaning because it encompasses a lot of different traits to it; level progression, character choice and interaction, hard rule-sets that emulate tabletop settings or create customized rulesets for their own game, a dose of exploration and discovery, player interaction and storytelling, to name a few. Combat can be boiled down to different styles, progression and system mechanics can be tweaked, and gameplay can be fast, slow, or somewhere in between. It can be crafting, card based, party based, singular, monster hunting or catching, multiplayer or single player. 

A lot of these traits on their own are nothing, or are fully part of other genres in the gaming world. Combining them, however, with other traits gives us an "RPG" video game. It's like cooking, were adding the right ingredients to turn our food into something else. It is also why the term RPG is meaningless in a lot of ways, because it encompasses so many parts to it it's impossible to determine a concrete definition.

A shooting game is simple, concrete, FPS is first-person perspective, smooth aiming and a few other traits in design and gameplay, Third Person is over the shoulder, can have cover-based mechanics, and so forth. Another shooter genre is the run n gun the 2-d shooters. Those have no real argument to them.

So for Action-RPG, it's emphasis on fast combat primarily from what ive seen. Secret of Mana, for example, was an Action RPG because it had no turn-based mechanics, tactical combat, and level progression we normally see in an RPG at a time when turn-based was the norm. Likewise, Mass Effect, due to its combat mechanics, it is action-heavy, with power and gunplay use and squad commanding, but we see progression and dialogue choices that befit the role-playing game, along with a dose of exploration. that many latch onto as well.

What it lacks for other RPGs could be a number of things. Combat and rule-set come to mind, faster pace, different mechanics used for fighting, lack of total control of all of your characters. A more hack n' slash approach vs a tactical approach. We also can toy with progression as well, some Action RPGs use progression in the weapons you pick up, over leveling up stats. Some also focus on loot gains heavily vs character progression. It ultimately depends on the game, but when you say Action RPG, general traits are usually recognized, faster pace, some form of progression, player skill/control vs tactical control in many cases. 

This is why I argue the term has no meaning. It gains meaning when it is attached to that adjective. You are right, RPG does add meaning to the word. But RPG alone has none because it embodies too many traits, too many points that can make up the bulk of another genre.