This may sound counter-intuitive, but the combination of lack of use and being meticulous about over-cleaning any grill or smoker will shorted its lifespan. I have a Summit s-670 natural gas grill that is approaching 4 years old and my ex4 that is some 8 months old and both are in great shape. However, both are in use several times a week, and I leave the insides pretty much alone and the majority of the food scraping, grease and ash in place as it helps to coat and protect those mild-steel materials exposed to heat. But then again, I only infrequently use my ex4 at temperatures about 400F.
Like all equipment, especially those exposed to the elements, my experience has been the more they are used the better they both perform and hold-up. (My previous Summit was a 1995 version that I finally scrapped due to the unavailability of burner parts.)