I'm terrible about washing my cars, regardless of how old they are. I've had anywhere from a brand-new Prius down to used cars made 18 years ago. I think the most frequently I ever washed anything was the Prius, which I washed maybe 5 times a year. However, I think there's some factors that play into this.
We don't get much weather in the southwest, so it's rare that I even drive in the rain in the first place. I also have a garage, so dust and rain are generally kept off the car by way of it being in the garage. Lastly, I work from home now, so I don't even have to take my car out for commuting to work and back. Vaguely influential is the fact half my cars have been either white or silver, which hides dust very effectively, at least compared to my blue or black cars which show every single spot.
I have generally washed cars myself due to the fact that even though my time is technically worth more than the $25 it costs to go out and have a car washer do my car, $25 is still a lot of money to me when I can do it myself using maybe 10 gallons of water total and $2 worth of cleaning supplies. As for your concerns of rust, I think you may be mistaken here. Water doesn't cause rust, salt water does. Washing your truck regularly to get crud off it may actually help prolong its life.
Besides, if you live more than 2-3 miles from the ocean you're not going to be getting any of the salt anyway, so it's a non-issue. Inland cars in the non-snow parts simply do not ever rust out. I've looked at 1st gen Miatas from San Diego and Mazdas of that era are notorious for rusting out and not seen a spot of rust on them if the car was from the more inland part of SD.
I view this under the category of taking care of your stuff being very mustachian. If you view your car as a throw-away appliance, then not maintaining it or washing it makes sense. However if you want it to last and protect the resale value, you will need to wash it at least every other month in most parts of the dry southwest and wax it maybe once a year. Still peanuts compared to the overall true cost of car ownership in the first place.