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You know, unless the numbers go down for a while and then they double again.
Yeah, it wouldn't surprise me if wristwatches break this law in the future.
Unless the size of the market is also doubling.
Nope. 50% is 50%. It doesn't matter how big the market is, and that's Norvig's point. Once you penetrate 50% of the market, you're done.
He does admit that you can sell more than one item to the same customer, so you can have > 100% if you compare units sold to potential customers.
Indeed - maybe a corollary should be "in the same colour".
As pointed out by slazaro, this is incorrect. A decrease in usage and/or a change in the market size potentially lowers penetration as a percentage, which could enable it to double again.

As stated, it's either ambiguous or false. If it actually means penetration will never again double from the peak percentage it reached which was >=50%, it's trivial and uninteresting... if it doesn't imply that, then yeah, it's just incorrect.

So Norvig's Law is a truism?
Yes. It's useful for pointing out the fallacy of endless exponential growth.
The bubble always bursts.
I think the analogy would be a bubble that grows but never enough to pop.
This reminds me of "Herbert Stein's Law" which he expressed as "If something cannot go on forever, it will stop".
While this law is obvious, it highlights how we are so obsessed with growth.

Everyone has a PC ==> PC is not growing ==> The PC is dying. Everyone runs Windows ==> Windows isn't growing ==> Windows (and Microsoft) are dying. Everyone has a Facebook account ==> Facebook cannot grow much more ==> Facebook is stagnant and no longer interesting.

Maybe this is one reason why Facebook are so eager to buy smaller, growing social networks like Instagram and Whatsapp: It almost allows them to count the same users twice or three times.