The fact is, the battle is in the mobile arena now; the desktop market is still bigger and it still generates the most money, but it's also stagnant.
Opera wouldn't have been able to make any meaningful gains in the desktop market in the next few years. Even with the most optimistic estimates, by the time Opera was enough to matter on the desktop, the desktop market itself would not matter much.
The desktop doesn't matter anymore and Opera isn't doing great on smartphones
Opera Mini is the most popular mobile browser at the moment, but it's popular on feature phones. On smartphones and tablets, Opera has a minuscule market share, even upstarts such as Dolphin have more users.
If Opera is to stay relevant, it needs to be relevant in the mobile market and, unfortunately, that means switching over to WebKit.
There are plenty of reasons to do so, the two biggest being that WebKit is pretty much the only supported engine in the mobile space and that Apple doesn't allow browsers with their own engines in the App Store.
We've covered this in detail when Opera announced it was working on ICE, an experimental iPad browser with a WebKit core.
Whether they're good reasons and whether WebKit really is the only choice, is another matter. It doesn't actually matter much at this point, the decision has been made. Opera bet everything on mobile and WebKit. We'll see if it turns out to be a winning bet.
An Android browser is coming this month, an iOS one is in the works
Opera has a WebKit-based Android browser ready to be released at the MWC, a week or so from now. That's the only officially announced WebKit browser; Opera says that it's working on several others, the aforementioned ICE for example, but those are coming later.
Opera already has a smartphone browser for Android, Opera Mobile, which hasn't proven too popular. There’s no reason to think that a WebKit-based browser will do any better.
There are plenty of Android browsers out there and most people are satisfied with the default one. Opera would have to pull off quite a few miracles to get a significant amount of people to switch.
On iOS, the odds are even worse. Even if Opera creates a revolutionary browser, it will still be slower than Safari, since Apple artificially penalizes third-party browser performance. Users also won't be able to make it the default browser, and any link in any app will still open in Safari.
Opera has a tough fight ahead of it. Either Opera knows exactly what it's doing and has a great plan already thought out – in which case it may stand a chance, or it's desperate and rather than come up with something new it gave up the fight and decided to do what everyone else is doing.