Keeping the old interface was a good move, although it only (at least for me) leads to procrastination. The same problem exists, which is the mental adjustment that you need to make at some point.
Another interesting angle is that in the new interface, they had to add a dropdown menu to control the amount of padding used on UI elements (compact, cosy, comfortable). Had they gradually moved from compact to cosy, then to comfortable over a longer period, the need to provide an option might have been removed entirely. So some of the complexity avoided in having incremental changes over a fixed term has instead become software complexity needing maintained over a potentially longer or indefinite period.
In either case, a gradual transition would avoid the need for all this. For me, the two biggest gotchas with the new design was the text->icons change and associated relocation of some buttons, and the border change. Had the text->icons change been introduced distinctly, and allowed to stew for a few weeks - months, then the border size changed from compact to cosy for a few months, I possibly would have had no reason to complain at all (the amount of adjustment/effort required for each individual change is amortized over time, and by the time a new change appears, my comfort level has already returned to maximum for the current state of the UI).
