But unless you're playing in a league, there's no penalty for you yourself conceding if your opponent just leaves.
Really? You sure about that? So, you're saying if my opponent leaves the room for any reason, for any amount of time, and I concede, that's not logged as a loss? And my opponent doesn't get a win?
If that's really the case, then honestly I don't see why "rage quitting" is even an issue. Who cares then? It eats up no more time that a regular concession would.
The rules permit the use of horn decks, so why shouldnt it be legitimate.
Technically, the rules also allow "rage quitting." This isn't about what's legal, this is about sportsmanship.
That people play it is their choice, not yours.
And it's their opponents' choice whether or not to "rage quit" when they encounter that.
You are forcing people to not play a deck they want to play. If people want to play it, let them play it.
And if the people who play against it want to "rage quit," I say let them "rage quit."

Especially if the Horn player can then concede with no penalty to themselves, and not have to wait the ten minutes.
I find ragequitting much more childish than playing Horn.
They're both childish. That's the point: Childishness begets childishness, an eye for an eye.
Here's an idea for you, if you don't like the notion of forcing people to play (or not play) something they don't like: How about if people encountering a Gamling/Horn deck first request a mutual cancel with the opponent. If the opponent agrees, then the game is canceled with no penalty to anyone. If the opponent refuses, then "rage quit" and (according to Bib) the Horn player can concede with no penalty and no wait time.
Agreed. Horn may not be fun to play against, but it is legal to use.
Again, this is not about what's legal or illegal. Bib, would you really argue that using the Gamling/Horn/New Chapter combo doesn't constitute bad sportsmanship? Simply for its filtering qualities, if nothing else?
I hate site path manipulation, and it is very frustrating to encounter Courtyard Parapet or Doorway to Doom or Steward's Tomb several times a game, but it is still legal for that sort of thing to occur in the formats that allow it.
I hate it too, but I still see that as borderline compared to the truly broken Gamling/Horn/New Chapter combo.
Don't like it? Then don't play that format.
Why quit the whole format, when you can simply quit opponents using that strategy? There's still plenty of good fun to be had with many other players.