youtube-dl's main feature is its frequent updates, as soon as sites change their api/auth or anything. And that a happens pretty regularly. Therefore say youtube-dl from last month can't download videos from youtube this month, unless you update it.

Thus having a copy only saves you for a few weeks. Then it will start to fail on all sites, one by one.

Relevant. Youtube changed something that broke youtube-dl the day after it was removed from github. It couldn't update as the update source was from github.