When I was in 5th grade the Nintendo 64 came out and a friend of mine commented that this was going to be the last major video game console release of all time. After all, the graphics were so mind bogglingly good that no one would ever have any desire to create a better system. The N64 perfectly fulfilled everyone's game requirements.
Your argument is similar. Everything you have on your machine right now works fine as it is. What you aren't taking into account is that the list of things on your machine is defined by the machine's limitations. A new style of computation will open doors and allow things onto your machine that are currently not possible because of limitations in your processor.
Heh, the N64 was the first game console I ever used. I thought it was pretty spiffy. I still think it's pretty spiffy. In fact, looking at some screenshots, I would totally play some of those games over ones we have now.
Ignoring that, I agree with your point wholeheartedly: just because things work now does not mean we can't optimize. In fact, a very large part of the software field is all about optimizing things that already exist.