Very glad to see these back.
Also notice how Mearls is very careful not to imply that 5e will ever be released in PDF form.
I would be extremely surprised if they didn't appear in electronic format. Granted, that may not mean PDF, but it does seem a likely candidate.
In fact, if I were WotC, I think I would be looking at a way to set up some sort of subscription for these products - say you pay $7 a month and get either one $10 or two $5 PDFs per month. They could present that as an easy way to build up a library of classic products, where the price is low enough that most people could just sign up and then more or less forget about it.
But the real advantage of that comes when 5e is released, and WotC can then just add the 5e products to the subscription path. That way, the subscription becomes a
very attractive offering, and they can use the classic downloads to pad the "release schedule" for those months where no new 5e book is presented - in effect, the classic downloads serve to push 5e sales. (And anyone who subscribes specifically to get old edition material is also exposed to the new stuff.)
And if they could then tie the DDI subscriptions to D&D Classics subscriptions, possibly with a "buy both for $X" deal, all the better.
(They would need to look closely at price points and the required number of subscribers, of course. But since the subscription model seems to be their holy grail as far as selling the game is concerned, leveraging "D&D Classics" would seem to be a good way to make it work.)