Why is that even important? You're not playing cairne to break through taunts. You're playing it for how difficult it is to remove efficiently. Sky golem isn't even going to survive a round to take care of a sludge belcher (not to mention, even if it does, you have to remove the 1/2 or you lose your golem anyway, meaning sky golem is not cleanly killing belcher regardless). The type of deck that has a choice between these cards aren't benefited from higher attack more than they are benefitted from higher hp. If we're talking about a mech deck, the preferred choice is clear. Outside of that, cairne is a much stickier minion that is much more consistently going to do what you want, creating a board presence that is hard to remove efficiently.
The main reason cairne or sky golem isn't played, and even sylvanas to a certain extent, anymore is because they are too slow to impact a game you're behind in and lacking in versatility in games when you are even ahead. If you're ahead, your main goal is often to stop their strongest chance to win, which is usually some form of burst damage. These minions don't do that, therefore they've been cut for cards that do do that, like shieldmaiden for example. Even ET is going to help accelerate a win, reducing the chances they can get to their win condition.
As much as I like to run cards that work well into the attrition game plan, most top decks, especially in this meta, have much much faster gameplans and have no room for high value slow sticky 6 drops. And it has been this way since miracle rogue pre-naxx, almost consistently after the meta has "settled". So I think their decline has little to do with new cards but more of how people have approached the game on a more aggressive strategic level than anything else. And the control decks that might have ran these cards in the past have opted for cards that work to beat that aggressive strategy.