I don't think you thought this one through... if anything you're making the opposite point you're (I believe) trying to make.

I'm writing this on a macbook air that sizes up to <2.5% of the weight and volume of a desktop computer you're describing (screen, case and peripherals). It's also idling at about 2-3 watt, which is also <10% of the computer you're describing. It also produces much less sound, it's entirely quiet.

So size, weight and power usage and noise are way down.

The idea that I'd use a pentium 3 instead is ridiculous for these very reasons (heavier, bigger, noisier, using more energy), even in private use, and especially in public use.

It's also the reason why bigger, heavier, noisier and more energy-consuming cars, are also ridiculous to many people, particularly those not driving them and having to face them in the public sphere.