That maintenance has to happen one way or another, hiring someone to do it can add quite a multiplier to the price. For example, I was recently looking at water heaters and called 4 different plumbers to get a quote. All of them came in around $5,000 for the job. The water heater they quoted costs $1,000 retail at Lowes. If you know what you are doing it isn't even difficult to install.
That said a layperson probably won't know the new code requirements in their jurisdiction and if you sell your house you'll have the inspector tut-tutting the work for one reason or another.
Hah, although, admittedly, when my AC died here I was looking at quotes for $17K for replacement (although I did also do the furnace at the same time, since both were early 1980s and basic models even then). "Why? I could buy the AC for $4K".
"Surely it wouldn't be too hard to undo/redo piping etc." But yeah, different refrigerant, different code requirements for vents and exhausts and drains. 4 people working for 16 hours, I saw where the money went.
> If you know what you are doing it isn't even difficult to install.
How much time does it take to acquire & refresh the skills and code knowledge, and how many water heaters can you amortize that over during your life?
The skill is in being able to solder pipes together, something that is useful to learn as a general skill. It doesn't even need specialized equipment, just a torch, some flux, and a spool of plumbing solder. There is also a tiny bit of electrical work, but that's literally just matching the colors and using a couple of wire nuts.
The building code stuff is more of a bugbear.