Most Opera users are on mobile, not desktop. Opera is struggling on desktop. Sites weren't working, and that was the #1 problem with the browser. Fixing that will let them grow. Opera's got nothing to lose on the desktop. They can only gain users by doing this, even if a handful of hardcore users drop them.

Most people don't need most features in Opera 12, but they're just getting started with the new Opera. Now they can create a proper foundation instead of this:

http://www.favbrowser.com/opera-the-past-the-present-the-fut...

Opera 15 is not a fork of Chrome either. It's using Chrome (Chromium) as-is, but with a new user interface on top of it.

In other words: They may alienate a small hardcore group, but fixing site compatibility will more than make up for it because other people can finally start using it.