[variaga]

In almost all cases, switching processes means a complete redesign. 28nm vs 60nm isn't just "we can draw smaller parts now". Lots of other things (silicon doping levels, choice of metal for the routing layers, operating voltage levels, thickness of the insulating layers, ...) change too, requiring design changes to keep the old design working the same way it used to.

A very small set of processes allow one way reuse e.g. you can build a tsmc28hpm design directly in the tsmc28hpc process (but you can't build a 28hpc design in the 28hpm process) - but, for instance, neither of those is directly compatible with the tsmc28hpc+ (note the 'plus' in the name) process. And all 3 of those have the same feature size and are made by the same foundry.

[exmadscientist]

Also, 28nm pays a huge price in mask complexity etc over 40/65nm. So its cost per area is much higher. This works out to a win if your area gets smaller... or a massive loss if it doesn't. And if you just draw the same features with a different process, guess what, your chip cost just doubled (or whatever it is, depending on the particular processes) on the "more cost effective" process.