The whole point of a domain is to give a human readable way of accessing content. If you're going to resort to using numbers to access sites, you might as well use IPv6 addresses.
Even without internationalised domain names, using 6 letters, numbers and hypens (as allowed in the current system), then there are over 2.5 billion combinations per TLD. Then there are the 2, 3, 4, 5, 7, 8, 9 etc. character domains, and all multiplied by the number of TLDs.
For most sites there is no need to crunch up each page URL to be small, in fact there is an incentive to do the opposite and make them descriptive, especially for blogs. Only extremely large sites such as YouTube or MySpace need to be thrifty with URL characters.
On the occasions when small URLs are desired, that is when services such as TinyURL can be employed, but beyond that there is no need.
Additionally, using only numbers is very restrictive. A string of 10 numbers has just 10 million combinations. A string of 10 letters and numbers has 3.7 quadrillion. That's over half a million for each person on the planet.
