Why would Coke make anything other than Coke? Why would Chrysler make cars other than Chyslers?
Because they're trying to capture market share with people who don't necessarily want Coke or Chrysler exclusive of all similar products...but who nonetheless trust those companies.
If the product is sufficiently different sure, but having coke and new coke on the market at the same time - didn't go so well. While the systems may be different - WoTC would be competing with itself for marketshare of the fantasy setting.
That is the conventional wisdom, yes. But conventional wisdom isn't necessarily true. There are companies that manage to produce products that cover similar ground but still cater to different audiences- see the aforementioned Coke and Chrysler. Even within the RPG biz, you see that this is possible. Look at the original WoD games: while mechanical underpinnings were similar, their line of Mage, Vampire, Werewolf, Changeling, etc., were not balanced to be played with each other. Not well, anyway. (Later editions changed this.)
I've always seen the various whitewolf products as different supplements and facets of the same world - while not fully compatible (until, in theory, recently) they play off each other and use the same system. I think this actually argues against your point - If Whitewold had released totally different systems for each facet it would have been worse.
I guess I just don't see 4e as "different enough" from a fantasy perspective - it would be like releasing cars that look way too similar or soft drinks that taste barely different (maybe the ingrediants are different but if they combine similarly, who cares?)
As THE major player, the proverbial 800lb gorilla, WotC has the resources to pull this off...IF they properly differentiate the products, of course.
The question becomes what would they differentiate 4e to, they would have to completely rejiger the system to a very different bend or again, compete with themselves? Being unfocused here would cost them a ton of money.
If 4Ed were released as another product from "the makers of D&D", the rollout goes much more smoothly, since they wouldn't have felt the need to dis the previous edition, there wouldn't have been the same baggage about license cancellations (which may not have occurred), etc.
The rollout was a disaster even with the full force of D&D behind it, I can't help but think it would be infinetely worse if they did not have the D&D backing. I suppose you can say it was disaster because they were so busy running from 3e, but really that was just part of it, they needed a better marketing strategy from day one. And again, I think it would have been that much worse if they couldn't attach D&D to it (saying "from the makers of D&D" can also attach unwanted baggage "I already have D&D for fantasy, why do I need a foreign system?")
just some thoughts.