Most of that damage is from condensation, but that doesn't mean there isn't more damage you can't see. First, look at the 'gasket' around the glass. It's called a marine glazing boot and it's a U shaped channel that goes around the edge of the IG (Insulated Glass) unit. To keep condensation from pooling in it and degrading the IG seal it is perforated along its length and has big gaps where it is cut to bend around the corners. All of the condensation that occurs at the edge of the glass either spills onto the surface or is dumped into the groove plowed in the sash rail to accept the glass. All the water dumped into the sash has to migrate through the wood to dry and vapor drive takes much of this to the back of the cladding. Marine glazing is cheap and easy to manufacture so it is prevalent on builders grade windows. Think Malta, Crestline, Craftline, Vetter, Caradco, LP, etc. Second, look at the glass. The airspace is thin. They might be low E, but they would be made with cheap old tech coatings, and the slight airspace means little or no argon. Or they might be plain old Insulated Glass. Either way they are inefficient by today's standards, which means there will be more condensation. Now look at the weather seals. You can't see any in your pics but there are typically two in windows like that. The one around the perimeter of the sash is called a dust shield because it's primary function is to keep debris out of the concealed hardware. It provides very little in the way of draft protection. The second is on the frame and it is typically a very small hollow bulb or a tiny vinyl fin. Neither is very effective and there tends to be large gaps at the corners. They seal just well enough to allow water vapor to condense on the glass rather than escape to the outside. Some small condensation events can actually be resolved by simply unlocking the windows, loosening the weatherstrip enough to allow the moisture to escape before it condenses. Finally, look at the cladding. On most of the windows I mentioned above the cladding is roll form aluminum (think seamless gutters, but much thinner) that gets clipped onto the wood sash rails and held in place with adhesive. In order for the rails to fit together the corners of the cladding are loose enough to slide past one another, which is loose enough to let wind and water and other stuff in too. The adhesive is a vapor retarder and we all know what vapor retarders do. When you see one of these windows with loose cladding it's usually because the wood has rotted out from behind the adhesive, often long before they look like the windows in your pics. Now you understand why thy look like they do. Use that information to identify builders grade products and advise your clients that these things have a life expectancy of 15 years or so. They can spend lots of time on maintenance or replace them at regular intervals. Premium wood window products have much better water management details and may last 20-25 years.