That feels pretty labor-intensive. I think there's probably a chance they could explicitly throw open the door to DMs Guild supporting all the pre-5E lines, though, which is a very easy thing for them to do.
Yes . . . which is why I won't be holding my breath.
But . . . offering full support for classic books could build customer confidence in purchasing NEW books on D&D Beyond. We've had folks in this thread still salty (rightly so) about losing access to their 4E digital investment, this pushback would (mostly) disappear.
And . . . that's a lot of titles! They should all be available as individual book purchases (micro-transactions?), but it would strengthen a full-on subscription option. Pay $15/month and get full access to current AND classic titles! And maybe earn points (meta-currency) towards purchasing individual titles permanently.
If WotC could make a recurring subscription model exciting . . . like Hight did for WoW . . . that could be a significant source of revenue in this new digital age.
But in my dream scenario, all titles would still be available for individual digital purchase in multiple formats (web, PDF, EPUB) and also in POD (print-on-demand). And yes, open all that up on the DM's Guild! All the editions, all the settings!
Labor intensive, yes. But forward-thinking and franchise-building.
Yeah, it's not going to happen, I know.