Well, wet dreams could relieve the buildup of sperm. And eventually the reproductive organs would learn to make less. Right?
Yes, wet dreams is one way the body gets rid of excess semen. Not everyone sleeps completely through a wet dream, but it does provide temporary relief. "Temporary" because the seminal vesicles continue to produce semen.
If you are not constantly arousing yourself, then, yes, the rate of production does slow down a bit, but it is relative. What once required relief every other day might slow down to once every four days. The problem for young men is that your hormone levels are high and fluctuating, so the rate of production doesn't stay steady until you get into your twenties.