I believe I'll chime in here.
What's being suggested is, for any large company (WotC, etc.) financially impossible. Not difficult. Not tricky. Impossible.
These are the facts on the ground.
1) The fact that a book is electronic does not mean it can be increased in size without cost. Writers still have to write the extra words. Layout takes longer. Editors and developers still have to go over the extra words, and check the new mechanics. (Even if they're not tested "to perfection," even a bare minimum of testing takes substantial time.) And yes, you'll need more writers and more developers, because nobody is an expert in all systems. You're talking about substantially--not slightly--increasing the price point.
2) You're increasing the price point, but adding material that's only going to be useful to a fraction of the audience. Yes, there's a large population of gamers who play older editions. It is not a large enough population to make up for the added costs. You will not--not might not, will not--attract enough new buyers to account for A) the extra costs of production, and B) the sales lost from players of the current edition (whatever edition that is at the time) who choose not to buy the adventure because it costs so much for material they aren't going to use.
And honestly, you're not going to get even a small fraction of the old edition players, either, because many of them are quite happy with what they have, or have other companies to buy from, and they aren't going to want to shell out big bucks for an adventure that's half useless to them anymore than current edition players will want to.
3) You are vastly underestimating the amount of work it takes to modify an adventure from one edition to another. It's not as simple as saying "Replace monster X with monster Y." Some monsters are of such different power levels, or have changed in such dramatic ways thematically, that they're simply not comparable between editions. Same with magic items. Same with the assumptions about PCs.
3a) The games are built around different assumptions as far as monster difficulty. 3E is built around CR--how dangerous is this creature to a party of five. 4E is built around monster level--how dangerous is this creature as one-fifth the opposition faced by a party of five. And that's just 4E to 3E. You start going back even further, and the mechanics become even further apart. It can, in some cases, take almost as long to convert an adventure (if you're doing it to professional standards) as it can to write a whole new one.
No, I'm not exaggerating in the slightest. I've done it for WotC.
On occasion, WotC publishers a conversion, where someone has taken a 1E or 3E adventure and made it 4E. And each of those is contracted as a new assignment, because it involves mostly rewriting, not the tweaking of a few numbers.
And this isn't even touching on the fact that, for major companies like WotC, adventures are among the least profitable of a line that's already running on razor-thin profit margins.
As an occasional gesture? Sure, the occasional adventure can be made for multiple editions. And of course, the closer the editions are to each other, the easier it is to do this.
But as a regular policy? It would cost WotC far more in extra contracts and lost sales than it would gain them in new sales.