So what makes you think you need to replace a smoke detector every 5
years? I had units in my house that lasted over 20 years.

Have you actually tested it with (real or artificial) smoke, to make
sure that the sensor still senses?

The test button does not test the entire sensor chain, so you can have
a smoke detector which passes the button test but fails to alarm on
actual smoke or ionization.

Most of what I see on the Net says that replacing smoke detectors
after 8-10 years is recommended.

Likely a dusty environment.

Pretty common problem.

The Americium in ionization-type detectors has a very long lifetime,
but dust and debris and etc. can cause the sensitivity to degrade.

On the other hand, I have not seen a CO2 detector for near the price of
a smoke detector. Are they really so inexpensive?

You mean CO detector.

My detector rolled over yesterday; full of dust and crap.

In Lowes I found battery powered, stand alone CO and smoke detectors
starting out at about $20, battery powered, stand alone combo CO/smoke
detectors starting at about $50.

My understanding is that the sensor in the CO detectors, being
chemically based, does have a limited lifetime. As of 2009, ANSI/UL
specs require that such alarms begin chirping an "end of lifetime"
signal after 5 years of operation... and a couple of weeks after this,
you can no longer turn off the chirp.