Was really hoping that Horses would be like Castles and Knights (which would also have a lovely thematic benefit), but I guess in Dominion, a Horse is a Horse, of course, of course.
Livery is proof that we will never see a card that increases the cost of things, as otherwise it could gain the entire Horse pile.
If a cost increaser (let's name it "Highwayman") were a Night - Duration - Attack, it could read something like "Until your next turn, cards cost $1 more. For the rest of your turn, the only card you can play is Highwayman". You could instead make that second sentence "Your turn immediately ends", but then this attack is only marginally different from Bridge Troll's attack. With this wording, Highwayman's attack is to Highway as Bridge Troll's attack is to Bridge.
Anyway, that text
should prevent you from getting all of the Horses, even with Outpost or Mission.
That way, the only person who can gain the entire Horse pile is an opponent you attacked, and you would have nobody to blame but yourself for playing a cost increaser Attack in a game where Livery is in the Kingdom. The only counter example I can think of is that you could get all of the Horses by playing Possession + this hypothetical cost increaser, assuming the possessed player has Liveries in deck. Even then, either player would have a chance of getting all of the Horses. You may go through their entire turn, find no Liveries, then draw a Livery for them in clean-up and secure them the 30 Horses.
Nevertheless, like you, I'd also bet on cost increasers never existing. The -1 Coin token accomplishes a similar enough idea.