I agree with everything you said but this. There is so much content from 21-85 that never gets touched now. Some of it seen by such a small percentage of the player base at the time it was current.
Yes, there is a lot of content, but the amount of content doesn't really have anything to do with why F2Ps exist. Back when trial accounts were implemented, the level cap was 20. You were given 10 days to try the game out, and if you made it to level 20, you had to buy the game to keep playing past that 10 days. The only thing which has changed about free trials is they removed the 10 day restriction and called it a 'starter account.'
Meanwhile, they've added BC and Wotlk to the battle chest and will probably add Cata to it as well once the next expansion comes out.
I'd love it if Blizzard expanded the trial f2p model to be based on expansions. Leave it as is for free trial, charge a one time fee to unlock the rest of vanilla, then a similar fee for each non-current expansion. Also have small fees to unlock other trial restrictions.
Most of Blizzard's revenue comes from the sub fee. Box sales are only a small portion of them. Giving you more ways to avoid paying for content is not a priority, considering nothing about the trial account has changed other than removing the 10 day limit. You get as much time as you want to access the first 20 levels of the game within the restrictions put into place. If you want more than that, you have to pay for it. Blizzard sees the starter account as a 'free sample,' where you get just enough to want more, then you come buy the full game.
I really don't see that changing anytime soon. I also don't see why they would give you a bigger free sample.
The vast majority of the paying subscriber player base is focused solely on current content or getting toons there as fast as possible. Why not let others smell the roses that few stop for now?
I don't disagree that it would be nice, but you have to understand they aren't going to give you more content without making you pay for it. Sure, they'll give it to you at a discount by merging an expansion with the battle chest, but they aren't going to give it to you for free. If you want more content.
Like I said previously, the only way the game would go F2P is if their subs dropped low enough to where they would have to dramatically alter their business model to retain profits.
When that happens, they'll probably just make Wow Buy to Play and remove the sub fee, but also stop development on the game.