Wow, you're incredibly far off.
If you think a PC is more powerful than a mainframe, you're... I don't know how to even start a rational debate with you.
The idea is that mainframes DIDN'T GO AWAY. They're still there, and they're much more powerful than any desktop PC. It's just that the vast, vast, vast majority of people don't need that power.
Just like the majority of people using PCs today don't actually need all that power.
Mainframes are just made up of many many PCs, such as the IBM blade server. The CPUs are designed specifically for server use, but there's no magic to it. It's just PCs.
Google uses a network of off-the-shelf hardware, with some custom chips thrown in when they don't need all the stuff on the motherboard. If you think mainframes are some mysterious refridgerator-like boxes containing "supercomputers", well wow - then you're incredibly far off.
There isn't really a tradeoff in power/performance between one desktop CPU and another, there's just design differences. They don't rely on being very job-specific, nor power saving.
They don't have to run in an all-in-one tablet, which is where some serious tradeoff of power for size is going on.
In short: you are wrong, in an epic way. What do you think computers are? Fairy dust machines made magical by Steve Jobs??
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Exactly. You don't need more horsepower to replace yesterdays PC anymore. Because the majority of PC users stopped needing more horsepower several years ago.
Actually you're wrong on that account as well (and I guess now you've seen the light with the iPad.. I'm surprised it took you so long, considering your über-positive Apple-do-no-wrong attitude!)
It's not replacing PCs, it's offering people something with a fraction of the performance of a PC, but it can run Facebook on a glossy touchscreen, more or less idiot proof.
It replaces a PC like a tricycle replaces a motorcycle. (i.e. not really, you're just getting something idiot proof, with a fraction of the performance and more lightweight, but I guess most people can get by on a tricycle for minor trips)