(1) The actual retro market is incredibly minuscule compared to the existing D&D and Pathfinder markets. Most of that market is incredibly conservative and you're never going to make any appreciable inroads in picking them up.
If you build it, they will come. I don't know the numbers that would legitimately compare the people PLAYING older editions and retroclones versus those playing 3E/PF, and those playing 4E. I don't believe ANYBODY really has those numbers. Oh, you can compare SALES but that's not the same as knowing how many people are actually playing what since older editions aren't going to show up in current sales figures and even with 4E you don't have to own and continue to buy tons of materials to continue to play.
As you say though, that market IS conservative. But that doesn't mean they won't go for anything new - it means the game they want to play needs to appreciate and accommodate the STYLE of rules and gameplay that they're after which is a LARGE part of the discontent with newer editions. It's not a blanket disdain of anything new - it's a desire to get back what has been lost.
(2) 3.5/Pathfinder gamers generally like the fact that 3rd Edition cleaned up AD&D and incorporated most of the house rules people had been playing with for years or decades. That's why they're playing 3.5/Pathfinder instead of a retro clone in the first place.
Even for those for which that's true it's FAR from being their ONLY reason behind their choice. If I had to guess (and I suppose I do since that's sorta what the thread is about) most people are likely playing 3e/PF largely just because 3E is the version THEY started with.
(3) 4th Edition is a radically different game and a slightly tweaked version of AD&D is not going to appeal to 4E fans.
So you'll lose 4E fans (probably in droves). You almost certainly won't pick up any significant portion of the 3.X market you've already lost. And even if you created an OSR-darling, the conversion numbers would still be minuscule.
I don't think you're seeing/appreciating the larger picture. It's like you refuse to believe that people CAN change editions for some reason - but it happens a fair amount. Certainly it happened on a MASSIVE scale with the change from 1E to 2E, from 2E to 3E, from 3E to 4E and over to Pathfinder. People have drifted around after all these changes, whether back to their originally experienced version of D&D, over to other non-D&D systems, skipped over editions to land on playing a more recent one, etc. I've seen any number of people posting to say they missed 3E altogether and now love 4E. I've seen people who moved back to 3E from 4E, or to retroclones. People moved from 3E to PF and never touched 4E at all. Etc. Etc.
Someday 4E will end - that is to say that for one reason or another WotC will almost certainly cease publishing it/actively supporting it. Or maybe a business model can be found that will allow them to
stop pretending that other versions of D&D universally suck or don't exist. People can and do play multiple versions of D&D and can and will drift from one to another as their own preferences - and the gaming market - shift.
But it probably won't because OD&D is an unplayable historical curiosity. I think every serious gamer should hunt down a copy and read through it because there's a lot of valuable insight to be had from that, but it has basically zero mainstream appeal.
See, now this tells me only that YOU have a curiously narrow point of view. "
Unplayable historical curiosity"? DUDE... the game you play NOW - regardless of what version of D&D that might be or even IF it's D&D at all - owes its existence to the fact that OD&D was extraordinarily playable and enjoyable leading to so many people expanding on what they could and would do with it. Yes, it's quite... simplistic compared to a "modern" set of RPG rules - but that's hardly the explanation for why it's now such a small portion of what's actually being played.
"Unplayable"? That's really a rather boggling word to be using.