It sucks, and they really should remove it from the store well in advance, BUT:

Does anyone have any reasons why I shouldn't get my gold back? I haven't used any of it. Not a second of the perks, not a single arena entry.

The reason why would be that it should have been obvious that you weren't buying the new pass. The fact that it was still the discounted price was a huge red flag. Didn't the purchase page also specifically cite that the pass would only be good for the Outlands expansion cycle? It sucks that the first transition wasn't smooth, but it's not like you clicked on a pass that was the right price, or a pass that said it would work, you clicked one specifically stating it was already expired and basically bought 4 expensive arena tickets.

Don't get me wrong, you got screwed, but in the end if they don't want to fix it, you can't really do anything. You didn't spend real money, and coins have no actual value to demand back. They could randomly delete your entire account on any day they feel like, and there's nothing you can really do if support says "too bad" other than charge back purchases, but that only works for real money purchases.