OK, I'll say that it depends on the driver.
Some like a stiffer ride while others do not. I do not, at least not in my 18 yr old Volvo with OEM struts/shocks. My car's not a sports car or hot rod so I want it to have a nice smooth ride, not feel every bump in the road. I have another car that is a "hot rod" type and has everything high-perf but it's not a car that I'd like to drive on long trips as it's not that confortable.
My Volvo is confortable and handles as well as expected. I've added better IPD sway bars, sway bar links, strut braces BUT the springs, struts and shocks are all OEM. Volvo struts and shocks last a VERY long time (EXCLUDING '99, '00 S80s) so for cars before those days, I'd agree with your mechanic in that front struts can easily go past 150-200K miles. They aren't bad unless the car bounces too much, binding, noisy (although usually something else) or leaking oil (rare with Volvos.

Another note, many times I've read here where people have switched to Bilsteins and were not happy afterwords. They often complain that the car then rides too stiff and/or has new noises afterwords. I'd research the matter before jumping into it and if you opt for Bilsteins, keep your old stuff in case you'ld rather go back.