There is a huge difference between washing your car, driving your car in the rain, and or course, driving your car on salt-laden streets in the winter.
Having once owned a car wash, I saw hundreds of cars each week come in for washing. Some were regular customers washing all the time, some only rarely. There were many times when I'd see the same era cars come in, (say, late 1980s models) that were rust buckets, and or blistering paint, and the same models come in that were in excellent condition. In all cases the best ones were regularly washed, and often garaged. The worst were ill maintained and in some cases, parked under trees. Some people cared about their cars, others treated them as a transportation appliance, used them until they stopped and then got rid of them. Everyone is of a different mindset.
My wash offered complete self serve as well as a touchless automatic. We didn't see too many roadsters come by, but some did come for a hand wash they did themselves. We did see some customers driving a very long distance to my wash, for we could wash in the touchless an F350 4-door Supercrew with extended 8' bed, duelly wheels and a 6" lift kit (or the GM/Ram equivalent); no other touchless in the area could do that. They loved their trucks in Michigan.
One little old lady would always wash her car by herself, but as a courtesy (it kept her coming back) I'd turn on the powerful dryers in the automatic, and she'd drive back and forth until her car was dry...and she'd be back the next week. Another older man with handicap plates would come by for an automatic wash, and was grateful I'd wipe down his door seals by hand and spray them with a silicone spray to prevent freeze up in the winter. He was so grateful.
It would be extremely unlikely that you would cause your car to seriously deteriorate [in your lifetime] by washing alone. Driving on wet pavement [or worse, in the rain] will force or push all kinds of road grime deep into the recesses of your car; washing will get a lot of that out if you are careful. When there is salt in that grime, that's when trouble begins. In the winter, those salt-laden streets are serious trouble. The older "classic" or collectible cars (of which there are hundreds of thousands in Michigan) would hibernate from October until the "April showers" cleaned the streets of salt.
I always gave my car a couple of coats of wax a year, always did a clay-bar and polish before hand. Gentle washing (never needed high pressure) and that's it.
