Why? What's with this relentless effort to downplay what people can expect in these threads? There was never any reality to the idea that there's a real economic incentive to artificially exclude discs from this type of feature, and the Xbone BC feature that's been revealed in the interim should make that even more clear. The PS3 played PS1 discs and it had absolutely zero effect on the level of digital Classic sales -- the same would be true here, some people still have collections they might dip into but many people would still give a shot to older titles for the very first time via download.
In a world where a disc feature is released, this is pretty much what I'd expect -- disc games would always just run in normal or very slightly improved (a la PS2 -> PS1 support) fashion, while specific digital titles could be upgraded with improved rendering, trophies, etc. and sold digitally.