They are making significant changes to every single class, all the old subclass splat books are no longfer valid, and there will be a new phb to buy which replaces the old one. Sounds like a new edition to me!
Nothing is ever invalid in dnd, You can adapt and use content from anything ever released it just takes work.
As far as if it's an edition or not, it's wotcs own view that this is not a new edition just an update of 5e, so if anything it's more of a 3.5 situation
Yeah, I mean I take your point but equally you could adapt 3.5 content for 5e. Personally I think Wotc are trying to manage the optics more than anything else and this is really '6e with good backwards compatability' more than anything else.