Very good points. These days the improvement from one release to the next is smaller and smaller.
With the exception of 4K and a small speed bump, there isn't much need to upgrade.
Of course when we do, we want to make sure we don't update just before a new release.
Mostly because we are looking to keep that machine for the longest time possible.
Especially since Apple are generally making it harder for you to upgrade your own machine.
This is why I went with the build my own machine. It takes away the Apple upgrade angst. If I feel a need in a few years I can just upgrade the motherboard if there is a socket change and processor.
I just wish apple would licence macOS so I would not have to hack it.