However, there are practical considerations that mitigate the problems above:

Any search engine worth their salt can handle redirects, and will handle short URLs right as they become more prevalent. Plus, given that we search by keywords in the content, rather than URL itself, search engines will always be able to surface what you are looking for as they do today, regardless of whether the shortener is down or not.

Browser anti-phishing features and antivirus programs today handle unsafe links, detect spyware, etc. In fact, given usually we click on anchortext without checking or seeing the destination URL, this is not much different than short URLs. This level of protection will only get more comprehensive in the future.

As for user friendliness: these URLs usually look ugly and inscrutable, but I bet users will learn to live with them. In many occasions the site that display them could hide the URL and just say "link" or something visually friendlier. Another option is to pre-expand short URLs, either server side or through a browser extension. No big deal.

Philosophically speaking, short URLs are used for bookmarking ephemeral content. For quick sharing of huge URLs in email and now increasingly on Twitter as well. For sharing things in the now, relevant in the present. How often do you revisit ancient email or random thoughts on Twitter? It does not matter in fact if those links stop working -- over time, this old content becomes irrelevant anyhow. Most of it was short-lived to begin with.

In conclusion: short URLs are here to stay, and if anything, they should become easier to use, straight from the browser.