I don't plan on selling this house for many, many years. But what's to stop an inspector/realtor issue from arising years down the road? Shouldn't I just take care of this now to cover my ass? The permit fee is only around $100, I just don't want to open a can of worms because the new plumbing system is working just fine.
Cover your ass from what exactly? A home inspector looking at a well done supply system, done with modern materials? A home inspector has zero interest in who inspected what, years, or decades ago. Realtors are also a non-issue. The only time I ever saw an issue with this was in a town just outside of NYC, when a friend sold a house. In that bureaucratic hell-hole, you had to get a permit to resell a home, and the town building inspector would walk through, and search for un-permitted work. It was just a money grab. My buddy had done a full basement build out without permits, fifteen years before, and they "caught" him. They inspected what they could see, made him add two receptacles, and charged him a few hundred in fees. Big deal.
Whatever floats your boat. I literally explained how 95% of all competent DIYers would handle the situation. If you want to pay a fee, to have a government employee digging through your place, and coming up with God only knows what, in the way of real, or imagined issues, go for it. This reminds me of a neighbor who wanted to do a small job at his house. I gave him the exact advice I gave you. He didn't listen, pulled a permit, and dealt with a township inspector who was a total dick, made his life hell, and red flagged him a few times (at $75 per re-inspection, to look at imaginary issues) The same neighbor calls in a panic and says his inground septic system is failing, and there is sewer water pooling in the back yard. I then asked if he wants to listen, and fix the problem, or call the township again, and spend $20K for a new elevated sand mount system that will take up most of his back yard? He listened, I got a contractor to show up on a Sunday morning, with a small machine and discretely repair and extend his existing system, for about $800. You can lead a horse to water...................................