Unless I missed something, they are going to make the adventures card craftable. I would much rather have crafted the few cards I wanted than pay out 30$ for something I only played once to just get the darn cards. Personally, I am a big fan of this move.
I guess most people agree with you, otherwise Blizzard wouldn't have decided to go down this route. On the contrary, I play adventures because I like them, and I bear the frustration of multiplayer just to gather enough gold for the adventures (mostly once a week when the tavern brawl comes out, at least it's more fun than seeing the same decks over and over both on ladder and casual). If there's at least one more player like me that decides to try out Hearthstone for the first time after the change, they won't be able to experience Naxxramas and it will be a pity. The obvious solution would be the following: when an adventure goes out of Standard, they make it free and remove the prizes. People who want to try it out for the sole purpose of seeing it would be able to do that and it wouldn't affect their collection nor their gold.
I saw the original announcement and things aren't as harsh as I thought for those who already play: they say that adventures will be available forever for those who unlocked at least the first wing, so the plan is to gather enough gold to open the first wing of each adventure they release, and then open the remaining ones with no rush.