Doesn't this mean you never need to upgrade your computer?
Most people who buy Macs (an overwhelming majority) never DO upgrade their computer. The last time I looked into this, it was something north of 80 percent. And by "upgrade" I mean ANY SORT of upgrade, hardware or software. Once they've bought it, that's it until the day it dies or becomes intolerably awful, then they just replace it with a brand new one.
I mean, if software gets more efficient with each version and how fast it runs doesn't depend on how big the software is, we seem to have gone from just asking if Lion runs slower than Snow Leopard on the MBA 11 to invalidating buying more than one computer in your life.
"Software" doesn't always get more efficient with each version, just Apple system software does. I can think of other operating systems that pointedly do not.
OTOH, we have to remember that we are constantly asking computers to do more things. What you were able to accomplish with 10.1 is nothing compared to what you can accomplish with 10.6.
But mostly, I don't see what you're really getting at, your conclusion doesn't seem to make any sense. How fact A (software tends to get more efficient with each version) and fact B (the size of a program doesn't necessarily have much impact on how quickly it operates) lead you to conclusion C ... I can't see any connection there.