Richard,
Your points seem to define that NORMALLY springs outlast shocks.
The "core" of my question, however is: lets say I have a car with 90K miles on the clock. No cracks at all related to springs. So no NEED to replace. Now the question:
If I do replace the springs will I get a far better car (as it happens with shocks) or no improvement in comfort/handling will be perceived? (thus pointing to: don't change if it is not broken)
Thanks, Jorge