Treated posts in concrete will definitely rot off at ground/floor level if water collects, so the trick is to avoid that with proper grade/slope (which could be as simple as mounding concrete). It's a common problem because people don't think about the air/water interface and the problems it can cause with any wood.
I guess I have mixed feelings on changes in home construction. I started working with my dad, a builder, at age 10-12, and got to learn everything about building. A lot has changed in the decades since then. New homes can still have settling issues and all those problems that are related to wood changing over time, though some of the manufactured beam products minimize it a bit. Lumber now gets sorted and tested and marked for crowns/etc, so theoretically you can build to better tolerances if you buy the good stuff. There have been huge improvements in design for earthquakes, wind loads, and the like, to the point where wind loads that would tear the roof off a house from 30-40 years ago won't budge it on a new home (before hurricane clips and structural hardware, trusses/rafters were toe-nailed in with a few nails and that was it -- virtually zero uplift resistance other than the weight of the materials). So in general, I think the advances have been beneficial.
As far as putting additions on houses, it all comes down to how the footings and supports are engineered and how they can be re-engineered. I don't think that basic truth has changed much over the decades. Perhaps the footings and supports in older homes were overbuilt for lack of engineering. That was the case with a lot of stuff back then.
