All true, of course. In fact, new technology is the wild-card in everything I wrote. Even if you were to buy a machine and leave it in the box, never use it, and later offer it for sale as a brand new item, it's value would eventually decrease to zero. What that means, in terms of real world costs, is that you need to think about how advanced a machine is, or how/where it fits into the flow of technological progress, in addition to just its physical quality and life expectancy.
In fact, you almost have to think in terms of planned obsolescence. Not planned by the people who design and build machines; but planned by you, right into your business strategies. My own view is that if something continues to do what you bought it for, then it's never really obsolete, even if the guy down the street has something newer and fancier. But as a market commodity an and asset value, machine tools age pretty quickly. We're all familiar with those dear and special machines that literally last forever. Old Acme's, for example, or P&W Jig Borers. And I know shops that still make money using those machines. But to sell them would be preposterous. They're not even worth scrap value, once you unplug them. So, to buy a machine whose physical life is longer than its financial life is to plan on keeping it forever. Long before it wears out and stops working, it'll be valueless to everyone but you.
Planned obsolescence in your business stategy means deciding in advance what a machine's life cycle will look like, and buying, using, and maintaining each machine in a way that keeps its productive value always in line with its dollar value. In other words, if you EVER plan to sell a machine, then you have only a certain window in which to do that. You need to keep the machine long enough to make it pay for itself, and to return the profit that you want from it. That's the minimum period of ownership. The maximum time you can keep a machine is determined by it's value on the used market. And that value will decline in time with BOTH physical wear AND technological aging.
As you say, your Mazaks could be kept physically young for a long time, for less than the cost of replacing them. But they could easily be ancient in the marketplace before you ever recover your rebuild costs.
And, of course, even if a machine continues to do what you bought it for, and is never really obsolete in your appliation, you still have to compete with the rest of the world. If your sweet old machines can't do their jobs as well or as economically as their newer counterparts, then keeping them busy will get harder and harder. And, there's never any guarantee that the work you originally bought them for will actually be needed in the future. Technological change drives your customers, too. You don't want to be perfectly equipped to run .005" tolerances out of
12L14 bar stock, when the world is looking for .0005 tolerances, and when leaded materials are considered to be poison.Life is change. Change is good. Ergo, life is good.
KG