This is an intelligent 5E discussion. Nice to see it after all the trollish ones recently.
It costs money to support 4e, even if there's no new content. It will cost a lot of money to develop 5e. Supporting 4e while rolling out 5e will canibalise 5e sales, at least to an extent. I just do not see it happening.
Remember one important thing. DDI was an experiment during the early days of 4e that went RIGHT. Just like Video Game publishers are moving to MMOs now because of the revenue stream, so to is WotC moving to a purely digital platform. If they can release new Corebooks that are Digital Only (DDI subscription required) they make a lot more money for a MASSIVLY lower outlay.
It costs a lot of money to publish books
It costs a lot LESS money to publish PDFs
No matter what, I do not think that D&D will exist much if at all in Solid Corebook fashion any more and if they can get everyone signed up to DDI - they can either
a) Stretch the 4e ruleset to anything they want over time, thus completely bypassing the need to release edition after edition
b) Release whatever edition they want in the future but do so purely electronicaly
Except of course, that PDFs didn't make WoTC the boatloads of money people claim they do, and that WoTC shut it down after a handful of months because Scribd was making more money off the pdfs than them OR Paizo were.
I like these broad generalizations of how PDFs will revolutionize tabletop gaming and how wizard is a fool for not trying them. It's like you exist in a world where they didn't actually try them.
Those -exact same- files are still being circulated today on torrent sites, etc. And that piracy doesn't even just impact the digital sales, they have a negative effect on physical sales as well.
Given WoTC's major profit-line happens to involve selling physical trading cards in physical stores, it's not impossible for them to want to support their physical meal ticket. Going all digital would have a negative impact on physical stores, which would have a negative impact on physical card sales, which would have a negative impact on their bottom line.
When you consider 'good business' ideas for WoTC... do remember that Magic: The Gathering exists. It's important.
It'd result in a smaller market, and less accessibility to the game itself from new areas of the market. Neither of those things are worth the benefits.