It all depends on where broadband is by the time serious R&D is going on for next gen consoles. If significant portions of some of the current consumer base (Australia, Canada, parts of the UK and rest of Europe) still have low bandwidth caps on their broadband offerings then I don't think DD can be the only method officially supported.
IMO, the next generation of consoles will be a transition generation where both Sony and MS push DD. The transition part is them somehow providing a way for users without uncapped broadband (speed isn't the issue, it's the bandwidth caps and surcharges after the cap is reached) to purchase and install the games while at the same time abandoning the 1 game = 1 media (Optical disks or game carts) method of retail distribution.
Perhaps it'll be like your suggestions (a good one, IMO) where the mechanism for 3rd parties to provide this service exists but the console manufacturers don't themselves do it. But that's something that is already built in as long as the systems use some form of account based game authorization combined with encrypted redistributable packages.
As long as that exists, then anyone and everyone could redistribute the games. A kid in the neighborhood could download it and spread it around to everyone else. At that point even bandwidth caps and surcharges on overage isn't a big deal as the kid could just charge everyone a dollar or something.
Or Gamestop or whoever can provide that service.
The potential roadblock is if retailers insist on being the sole redistributors of such files. At which point the security measures become more complex as each package would have to digitally signed to that person's machine or account. At which point you'd need a unique package for each person and sharing of files wouldn't be possible.
In other words, both are account based. But where a freely redistributable version can be installed on any machine where the account is flagged as having purchased the game, the retail restricted version would have the packaged signed so that ONLY that account could install the game.
Regards,
SB