We still play 1e/2e hybrid and that's not going to change. WotC can either get with the program and print stuff my group wants that we can use in our game or they will be ignored as they have been for the last ten years.
I like the idea of "editionless" books. I haven't bought any WotC D&D books in the 4e era. The last books I recall buying are the 3.5e Rules Compendium and 3.5e Spell Compendium.
However, I think this could be supplimented with some edition-specific materials in either an appendix, or web enhancements (preferably in actual printed appendices though, as making something a web enhancement puts it in a ghetto of being just some "online" source instead of in the actual published sourcebook. IME, DM's are less likely to accept something from any website, even WotC's, over material from an actual published book).
For example, the actual body of the book could be edition-neutral, but have an Appendix which has some AD&D 1e stats (since WotC is republishing the AD&D core books and still has a notable player base), an appendix with D&D 3.5e stats (as probably the most widely played out-of-print edition), and 4e stats (as the current edition, make it 5e when that comes out). It would also be an interesting way for WotC to show how to depict game setting material with different systems/mechanical toolkits.
The books would be crunch-light since it would only be several pages of appendices with stats, probably just some stats for key characters, maybe a few new spells or magic items, a couple of new feats or Nonweapon proficiencies, but enough that fans of that edition have a little something to localize the text to their specific edition, but it's small enough and isolated that fans of other editions don't feel that the book is filled with unusable material.