A lack of backward compatibility in both the upcoming Xbox One and PS 4 seems to be a big deal if you listen to the peanut gallery, but I don't think it really matters. Most game consoles have had limited backward compatibility, at best.
Of this generation, some very early models of the PlayStation 3 were fully backward compatible through the inclusion of what amounted to a PS2 system built into the PS3 motherboard. These models were quickly phased out, and on the models being produced within a year of release, you couldn't get even software emulation support for legacy titles.
In the case of the Wii, which had the best backward compatibility of all the consoles, support for GameCube games was dropped in subsequent redesigns. It didn't seem to hurt their sales one bit.
The Xbox 360's emulation support only extended to a limited list of titles. Microsoft was up-front in saying that only selected games would be ported. The advent of Games on Demand made up for at least some of the demand for older titles.
The success of Steam shows that the age of downloadable titles is here. I think that the PS 4 and Xbox One will be the last generation of consoles with physical disk readers. We're already seeing this trend in other media, with lackluster blu-ray sales, growing numbers of digital download movie purchases from Amazon and iTunes, and increasing numbers of subscribers to streaming services. I personally am no longer buying physical disks. I am also moving titles I still want to keep onto a hard drive, dumping the cases, and moving the physical disks into more compact storage. Some of those I may even re-purchase so that I have cloud access to them, making the IT administration someone else's problem.
Those who do want to play legacy titles are the kind of people who will jump through the hoops necessary to make it happen. For everyone else, it's relatively unimportant. If you can judge from the past generation of consoles, it appears that many cult or other noteworthy titles (like Psychonauts, which was almost impossible to purchase on a physical disk soon after release) will probably be made available for download in a format compatible with the new console, which will suffice for all but the most hardcore of gamers.
The whining about no backward compatibility is from a vocal minority. The vast majority of potential console purchasers won't give it a second thought.