A lot of people say the rusty sills are from condensation, I wondered if moisture in the sills can come from rain water entering the door via the window scrapper and travelling into the sill as moist air??
Unless the sills were completely sealed, with a vacuum inside, some moisture is inevitable. Look at the design of other cars from the time, and see how good Porsche's sill actually is?
You can't keep moisture out, so why not welcome it? Put massive vents through the car's sections to aid air flow, and plenty of drains at the bottom of sections where water can collect, such as doors, sills, boot wells etc., so the moisture can drain away freely, taking corrosive dirt with it The problem with 944 sills isn't moisture: the air is full of moisture and it's a natural thing that Porsche allowed for.
It's the breakdown of the protective coating on the metal that allows rust. This is partly due to age, but then would you prefer to take a gamble on a 1980s Alfa or Capri for perfect bodywork? The main issue is a build up of dirt, trapping salt as well as the moisture, and leading to the inevitable. Again, Porsche's design makes it easy to clean the dirt out: keep the drain holes clear, drive the thing in all weathers, and it'll wash the dust and muck out. How many 944s have spent a large part of their life being driven over salty roads, then put in a nice warm garage to keep the salt trapped for months? The later sill trim doesn't help here, of course.
One fault in the design is the rear corner of the sill, where it can't drain if the car is parked slightly uphill. I guess you can't design the perfect system, and after all the were meant to be used, not parked up!
The other advantage of the "open" design is easy access to check, and treat, the sills. They even made it simple to get a camera in, never mind a lance to wax the sills! If the worst does happen, they aren't difficult to repair (TVR chassis repairs, anyone? [&o]). Unless the rot has been allowed to spread for many years, or there is other damage such as bodged accident repair, it's a smaller problem on a 944 than any other comparable car from the period IMO, and a testament to the design.
It's lack of maintenance that allows sills to rust, or rust to progress, and as they are now out of warranty that's down to us!