Smoke detectors replacing - why? Lifetime of components?
As you suspected, apparently it has nothing to do with the radioactive material in those alarms that use it.

According to the Reliability section for smoke detectors:


  The NFPA strongly recommends the replacement of home smoke alarms
  every 10 years. Smoke alarms become less reliable with time, primarily
  due to aging of their electronic components, making them susceptible
  to nuisance false alarms. In ionization type alarms, decay of the
  241Am radioactive source is a negligible factor, as its half-life is
  far greater than the expected useful life of the alarm unit.


So it is just a matter of component reliability.  Although we are used to some electronic devices working for decades, we also accept that sometimes (but very infrequently) they just up and die, and perhaps on a statistical basis 10 years has been found to be a good marker for these alarms where one can be assured some large number of units (99% or higher?) will still be working.

Anecdotally, the house we are currently living in was built 20 years ago.  We bought it 10 years ago.  It has 10 smoke alarms in it.  I never knew about the "replace every 10 years" rule.  About two months ago, some of the alarms started to go off on their own (false alarms).  After this happened a couple of times, I replaced them all.