Opera isn't switching to WebKit, it's switching to the Chromium codebase (which uses WebKit). Chromium is a full browser, so Opera is basically outsourcing it's browser development to Google (which aside from practically all Chromium work also does most WebKit work these days).
This makes perfect business sense for Opera - why work hard to make a browser, when it can get one for free. As a plus, that browser uses WebKit, so its main competitors - mobile Safari and the mobile version of Chrome - will never be ahead of it on core engine features. Furthermore, one of those competitors, Chrome, will never be ahead of it in any way since they will be practically the same browser.
For that reason I doubt anyone believed Opera would be putting much engineering effort into core WebKit or even core Chromium. It makes no business sense - Google is already doing all the hard work on Chromium (and WebKit, with others), and anything Opera adds to WebKit shows up in its main competitors immediately.
What Opera does need engineers for is to replace the Google-specific stuff in Chrome, like login to Google services and other Google web services. Also Opera will modify the UI. Opera can also add some stuff on top of that to differentiate. But Opera simply no longer needs a significant browser engine department - Google is now that department.