I lived in Romania where until Chrome came along Opera was the second most popular browser (lagging behind Internet Explorer). People used Opera because other people used Opera. It was as simple as that. When you saw someone using Internet Explorer, you installed Opera and told them to use that. Then they saw how it was better, so they did the same with their friends and so on. Only the most technical users considered a non-Opera browser.
Opera is a pretty good browser, and long time ago it was the best browser. If you have a large enough population of Opera users, it's easy to see how it can remain sustainable. The more interesting question is how did Opera became popular in the first place. This a question this article and the previous one on the same subject don't address.
I loved Opera. I bought a license back when it wasn't free. The user interface was fantastic, it was fast, it had minimal, but usable e-mail, bittorrent, and IRC clients, and it came with a decent ad blocker. It also ran on Solaris and OpenBSD. It had sync. At some point Firefox came, which had the potential, but it was slower than Opera, and required configurations to bring it to my liking and extensions to maintain and install. I hated that. I switched to Chrome only about a couple of years ago when a new major release of Opera was really, really unstable on Mac OS X when using flash. I like Chrome because I don't have to configure anything, just like with Opera, but I still feel the order in which Opera displayed tabs was superior and damn I miss the 1-2 key shortcuts for switching between tabs.