There's a decent amount of other cosmetics that do the same thing already ingame. Valve kinda forgot about the whole thing.

ooferino, mind grabbing some examples for an old doofus?

Pyro also has his tank on his back, and the weapons are different, I believe even shotgun animations are different

for a new player, that's still pretty damn confusing, newer players might believe that they were fighting some type of special proto pyro.

You mention sillhouettes but it doesn't seem like you really know what that means. A character having a unique sillhouette is what makes them instantly recognizable without having to focus on smaller details. And if you look at the literal sillhouettes (ie outlines) of each of the TF2 characters, you can pretty clearly tell who is who just by their shape, body type, stance, etc. People seem to think cosmetics like this are problematic because they might confuse new players, but that isn't really true. If you compare the sillhouette of the soldier wearing this cosmetic with that of the pyro, you can still very clearly tell who is who:
https://i.imgur.com/EyqpBYu.png

The soldier is slightly taller, less hunched over, has wider feet, broader shoulders, etc., all making him clearly distinct from the pyro. This is pretty much true for whatever cosmetics you put on the characters and is one of the reasons that TF2 has such good character design.

if you see something the shape of a soldier, it's a soldier, that's what sillhouette means. There's way worse cosmetics that actually break sillhouettes but this item actually looks really good