One small problem here.
If I'm not a 4E player, why on earth would I pay /month for access to books that I already own in paper and electronic forms? Regardless of how inclusive the DDI was, if I'm not using the current system and getting new material, there is no intrinsic value in their software when I can own or can easily purchase all the old rules, supplements, etc. instead of leasing it with a subscription?
(Full disclosure - I'm one of the "lost sheep" that went to Pathfinder. But I'm interested in why you think this would work for any of the old versions.)
It works for 4E doesn't it? 4E players have books also (at least most of them

), and yet there are 60,000+ DDI subscribers...
If DDI had OD&D, 1E, 2E, and 3E support in the form of character builders, npc/monster builders, encounter builders, rules compendiums, and Virtual Table Top rules support (with import of character/monsters/npc's), and occasional articles/adventures in Dungeon & Dragon - do you really think that they wouldn't get a significant number of older edition players/DM's to subscribe?
I think they would.
And if they included Pathfinder support in the same manner? (Which they can, it's Open Content, not to mention mostly their IP to begin with...)
Of course they wouldn't get everybody, just like they don't get every 4E player and DM, but Holy Crap that would increase their subscriptions.
And not everybody has every older edition product that was printed or that they wanted. Look at the uproar when they pulled the pdf's from RPGNow. They could be selling them through DDI. That's a revenue stream that they are solely able to exploit, but are instead leaving on the table.
You might not be ineterested in that, just like a lot of other people that wouldn't be. But I'm betting there'd be more than enough simply due to the builders and VTT.
60,000+ (or as much as 100,000+ as Hussar proposed) times 5 or more (OD&D, 1E, 2E, 3E, Pathfinder, etc.)...
We already have pretty good anecdotal data that Pathfinder players are approximately equal in number to 4E players, and 3E/3.5E players aren't too far behind...that's a lot of untapped customers. I'm betting all told, that could push subscription into the 300,000 to 500,000 range. At aprox. $10 a pop, DDI alone would push the D&D brand close to that $50 million threshold to be considered a major brand (like Magic: The Gathering), before even considering book sales, pdf sales, and other products...
That's a whole different ballgame. A ballgame they currently aren't even suiting up for...
