I don't know why people think this is sustainable. HD collections this generation are trading on about four distinct UI/UX improvements between last gen and this one (SD -> HD resolutions, wired -> wireless controllers, memcards -> internal storage, nothing -> trophies/achievements) to deliver existing games in a significantly "improved" package. (And even so these things aren't actually selling so well that they're worthwhile for anything but already-huge market-proven franchises.) There aren't going to be any such improvements this next time around -- resolutions will be staying more or less the same, there's no pending upgrade in controller quality, etc. That removes all the incentive to rebuy stuff.
(You can look back at the music industry for context on this one. Vinyl/cassette -> CD was a huge transition for the industry because they were able to resell basically all of their content due to the huge convenience and quality upgrade that CD represented, but they were never able to repeat that feat.)
Now, it's certainly true that retail BC hasn't historically been a huge deal for people, but the more accustomed people become to the iOS/Android model for DD software where everything is seamlessly BC forever the more I imagine people will demand continuity on their DD software platforms going forward.