Backing up a few posts, regarding knowledge of the product line. I spend a great deal of time online researching, hanging out, etc. on the gaming forums, though I don't post all that often.
But I've met a number of gamers that simply don't go online for gaming forums and I'm sure where there's a few, there's likely quite a few (though I have no numbers). The point is, there are likely many who keep up with the latest by going to the store and seeing what's on the shelf, talking with their friends, etc. I've met people that didn't even know there's been online discussions about all this (hard to believe, I know). Time is limited. So those people could be pretty upset if there's not advertising at game and book stores.
There was a time that I took a break from lounging around the net, and I went out to buy 3.0. Before I could even read through the books, my friends saw 3.5 on the market and bought all the books, so I had to turn around and toss my books aside and buy all new ones. That really soured me on D&D. I got over it and played 3.5 (and other games, of course, like my Cosmothea RPG), but it was very annoying.
Then there are those that bought 4E, saw problems with it, then see Essentials out and are like, "Dude, I've already shelled out a ton of money on 4E!) It would be to WotC's benefit to make sure that places that sell their products have a poster or notification and then it's up to them to spread the word. I dunno. Probably no easy answer. I agree that they can't just stop selling Essentials and 4E. That said, it would be nice if WotC did as some companies do and give you a coupon for the upgrade come 5E, if you purchased D&D X period of time before the new version.