It seems people have a lot of faith in "WotC says it will be backwards compatible." I have to shake my head at that. 6E is over a year away, and WotC has a whole slate of products coming out during that time. "We have a new edition coming out, so these books have a short lifespan to them..." is not exactly something that will make books fly off the shelves.
Again, compatibility is something that TSR/Wizards has discussed since 1E became 2E, and with the exception of Essentials, it has never worked out.
I'm not saying it won't be an Essentials situation here, but that would be the exception and not the rule. And once again, an effectively backwards compatible game opens us up to a situation where products like Mayfair Games Role Aids or Judges Guilds Universal Fantasy products are quite possible, which TSR sued over in the past.