I needed a new water heater a few years ago. The local Home Depot had 3 types (I don't remember the exact numbers, but...) the basic model was $300 and had a 4 year warranty. The next model was $400 and had a 8 year warranty. The top model was $500 and had a 12 year warranty. I talked to a guy I know who does plumbing and heating. He told me that they were all the same water heater. The difference in cost was for insurance. I bought the cheapest one.

I should also mention that I have a water heater in the greenhouse that runs hot water through coils on a table to keep the plant roots warm. A friend gave it to me when he stopped using it for that purpose. He got it when someone gave it to him because they needed a larger one. So for me, it's third hand. I've been using it for 16 years.

I have bought several used tractors. I have never had a problem getting parts to repair most of them. I had a Deere model B, which was manufactured between 1937 and 1952. I had no problem getting parts. (Major parts were available from junkyards, but normal repair stuff was manufactured as needed, although frequently by third parties).

The first tractor I ever bought was new (1988). After 25 years it quit, and parts were not available. I talked to my local repair guy and his take was that repair parts are available for items that need repair frequently enough to warrant making parts to supply them. If an item is very unreliable, people quit using it, so they quit repairing it, so they don't make

parts. If an item is much too reliable and never needs repair, they don't make parts to repair it. You have to hit the sweet spot where the item needs enough repairs to warrant making spare parts, but doesn't need repair often enough to discourage people from pouring money into it. The number of units sold also factors into that.

I don't think you'll get more than 25 years worth of functionality out of a D810. But I think you'll WANT only perhaps 5, at most, before GAS (gear acquisition syndrome) sets in.

Lots of folks "just have to have" the latest model when it first comes out. Others skip every other release. The D800 came out in February, 2012. The D810 came out in July, 2014. The D850 came out in September, 2017. D800 users are probably drooling... or already have one, or have ordered one, or simply want one.

The digital camera market has evolved very rapidly since the first dSLR was introduced in the mid-1990s. About a decade ago, a new model came out every 18 months to two years. That has settled down to three or four years, but the new technology developments crammed into modern bodies (especially mirrorless cameras) are more and more impressive.

I think it's THAT sort of planned obsolescence (if you can call it that) that the repair guy was lamenting. The new model is always so good as to be irresistible, while the old one still works, and will for many years if treated kindly.