Li Shenron
Legend
Money matters, and to Hasbro (and thus to the success of D&D Next), money is the only thing that matters.
And financially there are three effective levels of fans. A casual fan who comes to the table and plays - and may have the core books but will have very little else. A customer - who picks out the books he likes on a case by case basis. And a subscriber who puts down money for products before seeing them (D&D Insider members and Paizo AP subscribers come to mind).
A subscriber is worth massively more than a customer - and a player who doesn't buy books is worth little more than someone who doesn't play at all. A subscriber is a direct income stream straight to you and it's a reliable income stream - it's not only more lucrative (especially as many also buy the books), it's much better for planning and budgetting. So you have vastly fewer overheads. A subscriber is therefore worth many times an oridnary player to the company. At present, two groups have demonstrated that they are prepared to become subscribers - 4e players and PF players.
For the sales targets, D&D Next needs both. An audience that was only as big as PF's won't cut it. 4e was outselling PF until they stopped producing material for it. So we can say that the fanbases are within an order of magnitude of each other. The question is what of the untapped fanbases?
Current 3.5 (and even 3.0 fans) need to be heavily discounted when it comes to potential sales projections - either is worth a whole lot in sales than a current 4e or Pathfinder fan. Both groups have demonstrated (a) that having a currently supported game is not relevant to them. And (b) they aren't prepared to put money in to get an objectively improved game - instead preferring to stick with what they know. And I do believe that PF is an objectively better version of 3.5 than 3.5 is. So by sticking with 3.5, current 3.5 fans are declaring that neither ongoing support nor an improved game are worth their money. This is not a market likely to buy a new and different game. Almost all current 3.5 fans are people who have out and out rejected buying books and upgrading to an actually improved system. To monetise them at all requires miraculously changing their gaming habits.
So regardless of how many 3.5 and 3.0 fans there still are, their decision not even to switch to Pathfinder points out that they are economically worth almost nothing. They aren't book buyers except in rare cases, and they certainly aren't subscribers.
3.5 and 3.0 fans should therefore be almost entirely ignored from a business decision perspective. Or possibly given adventures and splats occasionally. But they are largely irrelevant - they aren't even customers and are happy with what they have.
Which leaves 2e, 1e, and oD&D fans. We've already had a huge bite of the 2e lovers with a game that was meant to be just cleaning up 2e and giving it a more modern ruleset (3.0). It's an already heavily tapped well - and the 2e, 1e, and oD&D fans are used to not giving anyone money to play. After all there hasn't been a major new book for any of those games in a dozen years (unless you count e.g. OSRIC or Swords and Wizardry). Suddenly you need to change a decade's worth of buying habits and convince them to now pay a charge for what they used to have for free.
This isn't quite the mountain there was for 3.X holdouts. They haven't, after all, rejected a cleaned up version of the exact same game they used to play. They've rejected a different game. But even they compared to 4e and PF fans are a hard to monetise and unlikely to move to a subscription model.
Money matters (to Hasbro it's all that matters) - and there are only two groups in town showing willing to put their money where their mouths are. 4e and Pathfinder. Lose the 4e fans and you need to reverse brand loyalty and produce something PF fans prefer to PF when they have all moved away from WotC once already. Groups like the OSR may be loud - but when did they last put something on the best sellers list? Or indeed at all? I believe the Evil Hat (Spirit of the Century and Dresden Files) and Cubicle 7 fans to matter more financially. And both are small change.
Then there are the externals. People who currently aren't D&D fans. Say what you like about 4e, it had a plan to bring a probably receptive group in (that was derailed by certain incidents with Gleemax). An explicit goal of uniting the editions means they aren't even being targetted except as a side effect.
So what's WotC's big strategy?
I think your analysis is both reasonable and full of seriously arguable points at the same time.

You're first of all making one big mistake: you are missing the point that most people didn't switch to the next edition because they didn't need to. They just realized that they were having enough fun, or they had already fixed their problems with house rules, or they had spent enough money, or they already mastered the rules and didn't fancy restarting, and so on with more reasons... but this happens at every edition, and it is definitely going to happen for 4e fans too! Thus it could be said that 4e fans are not one bit more likely to switch to 5e than any fan of older editions, perhaps even less inclined since at the moment they might feel "strong" enough as a community that they can keep playing 4e despite WotC active supports is going to end.
Subscribers are also IMHO very often people who are fans of the gaming community more than an edition. They want the latest edition because being up to date is perhaps the most important or exciting thing for them. They probably like changing edition every few years (although they might get disappointed and grumpy if it happens too often). Clearly, it means they are 4e fans now, but among them there are also those least likely to drop D&D altoghter, because they go wherever D&D goes. In fact, if they spend more money on subscription than books, they also probably don't care about buying new books because that's what they do and like doing all the time.
Next, you are underestimating the amount of grognards (pre-4e fans) who can in fact be very interested in a new edition if they think (a) provides some benefits or refreshing new ideas, or (b) provides more people to play with. At least I am in such group, having stopped buying D&D books at 3.0 (with just one 3.5 exception) and having skipped 4e altogether, and yet I am very positive that I will start buying 5e books.
Then, you are kind of assuming that people who buy D&D books are always the same bunch. But most people started playing D&D when they were teenagers or young adults, and there is always a new batch of potential gamers that are not coming from any existing edition, and WotC needs to target them too.