I pulled out my old deck last year - it was over 20 years old and PT. I'd
never treated it with anything so it was well weathered, but no rot. It was
built about 15 inches off the ground and the back side of the wood was still
nearly as green as the day I put it in. (Had I known that I might have
pulled the deck up and turned it over years ago, but we were adding on to
the house so it had to go.
If yours has been in 20 years I'd guess it was either PT, or very well
sealed and maintained over the years, or you live in a pretty dry climate.
Plan untreated wood, particularly pine, would probably have rotted out long
ago. Cedar or Redwood might have lasted longer, but I don't think the
pressure treat either of these woods.
You seem to imply that PT wood lasts longer. I was under the same
impression until I read in builder's magazine that is not true.
The reason for using PT wood is to repel incects, termites etc. The
treating does nothing to improve the lasting aspect.
I asked about this at the lumber yard (not HD or Lowes) and they said the
same thing. Was news to me.