This was inevitable, regardless of how Mac sales are doing. The classic MacBook Pro design has no place in 2014's lineup, and you've got to get a $1700 laptop to a $1200 price point somehow.
High-resolution displays are great, but Apple's fetish for thinness is causing some unfortunate side effects.
I appreciate the speed and flexibility of TB and think it will pick up momentum as more and more people end up with ultraportables and other thin machines.