I see economy of scale as the key hold up here. A multi-source subscription that costs $10/month would be a no-brainer for a good number of subscribers but such an offering would immediately destroy the current subscriber base for publications like the NYT, Washington Post, Atlantic, etc. No one would continue to pay $10/month for one, when they could pay $10/month for all.
That would then mean that the $10/month all-you-can-eat plan would need to gain enough subscribers to offset the lost revenue from the old plan. Realistically it would have to result a 5 to 1 increase in new subscribers.
So you'd have to go back to an all-inclusive plan that is a discount over the individual plans but not a HUGE discount (let's say 20-35%). But then would that be compelling enough?