It sounds to me like they are sacrificing backwards compatibility. If you have an existing customer base this is one of the worst decisions you can make. When you release incompatible products you split your customer base along these products and if your newer product isn't sufficiently convincing then instead of gaining customers you will lose your existing ones.
Yeah I don't know the whole story but my understanding is that they originally made Unity work pretty well for mobile and 2d single player games so their 3d rendering and multiplayer support sucked. Now they're branching into the AA and AAA realm and they had to forego compatibility with their earlier rendering pipeline and multiplayer library in order to make it work for those use cases.
