It makes complete sense that YouTube introduced the rule because a ridiculous number of excellent URLs are wasted on inactive channels with only a few subscribers. By introducing the new /c/ URLs and using policies like the custom name being auto-generated based off your website domain, social media, etc. is perfect because it ensures that one build a consistent brand, rather than just trying to get a 'cool' URL.
The most important part of the URL update: channels with under 500 subscribers do not need a custom URL. I do think it's a little strange that they've not allowed new channels to create the old custom URLs, but I expect that's part of their effort to phase out the old URLs, in favour of Google+ URLs and the new /c/ URLs.