Sadly, I can't see it working.
Firstly, there's an issue that although there are lots of old-edition fans out there (as well as Pathfinder fans, of course), in a great many cases they simply have no interest in what WotC are doing. After all, if you never moved on from 1st Edition, there's no reason you'd necessarily even
know about DDI, never mind care enough to spend money every month on a subscription. Likewise, where the Pathfinder fans have moved on, there's a real good chance they simply won't look at
whatever WotC offer.
Then there's the issue of cost. If WotC want to offer ongoing support for old editions, then an article here and there in the magazines just isn't going to cut it. They would need to offer regular and frequent updates. Essentially, they would have to produce at least 3 (soon to be 4) magazines each month - one each for pre-3e, 3e, and 4e at the minimum. There could be significant overlap, to be sure, but even so that would be a
lot of material to convert. Given that the 4e board sees regular complaints that the e-magazines (esp eDungeon) are pretty sparce, I suspect tripling the workload is a non-starter.
(And, worse, they would have to start supporting the old editions, and publicise that they're doing so, before they could expect to see any real bump in subscription numbers.)
Finally, and bluntly, there's an issue of quality. Simply put, the OSR guys may well be better able to support the old editions than WotC (who haven't ever actually done so). Likewise, Paizo are certainly better able to support Pathfinder than WotC, and probably better able to support 3e (even if we have to convert the material ourselves). WotC have moved on, and the people who used to do that work are mostly now gone, so they'd have to either recruit or retrain people for the job... the OSR and Paizo have no such issues.
Now, all that said, it is entirely possible that I'm wrong. Maybe there is a big demand out there, maybe large numbers of people are eagerly dreaming of 'official' support of their edition of choice. My gut says, no... but I have been known to be wrong.
If I were WotC, and I were considering something like this as a direction going forward, here is what I would do: Produce a single,
new adventure module for 1st Edition. Do the best job we possibly can to make this a really good adventure, publicise it as heavily as we can, and watch sales. Then, if it fails, we know there's no great market; if it is a moderate success, we can continue with occasional support; if it is a runaway hit then we consider going for full-bore DDI support. (But the advantage of this is that producing a single product is lower risk than trying to do full DDI support, and if it does fail we can always redo it as a 4e/5e module and put it on DDI.)
The other thing that they probably should do is get those old edition PDFs up on DDI in some form. That would seem to be a pretty cheap thing to do (although some will need rescanned, and they'll need to do something about DRM/format/whatever), and it would probably bring in a bunch of new subscribers. Given that there's little to no creative work involved, just the mechanics of actually putting them up, that would seem to be low-hanging fruit.
(Oh, and incidentally - everything I said about old-edition fans not watching what WotC are doing applies equally to 5e. If they're banking on 5e pulling back huge numbers of 1st Ed fans, I fear they're going to be disappointed - if neither of WotC's previous editions provoked you to upgrade, why would you even look at their new attempt? Hell, many of them may not even know there's a new edition coming!)
