In reply to Rich Tabor.

How can the front end be the user's domain if core starts to decide which is the “right” way to create web layouts, with no possibility of change? ​

To me, what this kind of decision seems to imply is that, in reality, the front end is now core's domain and that anybody can be — or want to be — a theme designer, because all those boring details can be decided beforehand for them, with complete confidence and effectiveness.

If that's the case, this actually devalues a lot of the figure of the professional theme designer as we know it and will (possibly) remove a lot of nuance from this activity.