I legitimately cannot comprehend a world where someone believes this? Every Intel Mac ever made, and quite frankly every computer ever made, has gotten slower with each major update that follows.
There's different ways to slice that claim, though, which could explain why different people believe different things:
1. After purchasing a Mac, the
hardware itself becomes slower with each subsequent update to macOS released,
regardless of if those newer versions are installed on that Mac or not
2. After purchasing a Mac, the
macOS version it came with becomes slower on that Mac with each subsequent update to macOS released, even when those newer versions are
not installed on said Mac
3. After purchasing a Mac,
it runs each subsequent update to macOS more slowly than the one it came with
4. After purchasing a Mac,
your perception of its performance changes for the worse over time, regardless of if newer versions of macOS are installed on that Mac or not
...
Option 1 is untrue, but it's also a bit of a cheeky way to interpret the claim. The components of the Mac can of course wear down over time and thus perform worse than they did when brand new, but if your HDD starts filling up with bad sectors, for example, you usually say
the harddrive is failing rather than
the Mac is becoming slower.
Option 2, taken at face value, is untrue. If all you do every single day is boot the Mac up, run a "real workload speed test" and then shut it down, it's not going to become slower over time. It
can become slower due to externalities such as filling the harddrive completely and thus hindering the normal operation of macOS, but that's user error rather than hardware error.
Option 3 can be true, but it being true has nothing to do with
the original hardware becoming slower, and everything to do with
the intent and requirements of newer software becoming more demanding as newer hardware becomes more capable. This option, I'm guessing, is the most common way to interpret the claim by people who don't see or understand the distinction.
Option 4 can be true, simply because people have varied experiences. It's also highly personal, and not really something that can serve as a universal truth. One person might perceive their Mac performing worse because they're enamored with a newer Mac and
really, really want to justify that purchase to themselves (or their partner, as is the case sometimes). Another person might have graduated from browsing the web to editing daily 4K videos of their offspring, and the Mac that used to do just fine now suddenly feels slow.
Did I miss an option?