do-release-upgrade -d Ubuntu 13.10 -> 14.04 failing

This answer is almost off-topic, but something what I've been wondering.

Every time I upgrade Debian or some of its derivates, such as Ubuntu or Mint, I just do

sed -i -e 's/olddistroname/newdistroname/g' /etc/apt/sources.list
apt-get update && apt-get dist-upgrade
reboot

And then I enjoy my new, just released distro.

This has worked for me since forever. All this do-release-upgrade hoopla makes me a very confused, angry, old-beard.

So my sub-question is: what is the benefit of those do-release-upgrade style commands if they don't even work? What's wrong with the proven, working way? (apt-get versus aptitude is another fight I'm just trying to digest)

In my case, the same error was caused (as suggested in the error output) by the repositories for my release (13.04) being unavailable. This in turn was due to 13.04 just not being supported any longer. (You easily notice this by the failure of apt-get update.) It seems like in such a case you can not do-release-upgrade but actually have to add some repositories which provide the new packages (e.g. just replace old by new release name in /etc/apt/sources.list) and then apt-get update && apt-get dist-upgrade as suggested by Janne. (This way also the package signatures are verified, of course.)