@bondsbw: An interesting read here, http://my.opera.com/haavard/blog/2013/02/13/webkit, on the (unofficial) blog of an Opera employee.
tl;dr: It's too difficult to develop a rendering engine with a low userbase when the web is too closed/lacking on standards compliance.
Presto was actually designed from the ground up with compatibility in mind. It was always a goal to be compatible with the real web while also supporting and promoting open standards.
That turns out to be a bit of a challenge when you are faced with a web that is not as open as one might have wanted.
Yes, monoculture is bad, but Opera was never really in a position to prevent it in the first place. Even with Opera as the dominant mobile browser and more than 300 million active Opera users in total across all platforms, web developers still designed just for WebKit.
But it's not all bad. He contends that switching to WebKit will enable Opera to innovate more, polish the browser, improve WebKit and the web in general and help Opera push the web further in a direction that competes with the closed world of native apps.
Still, Presto you will be missed. In Trident and Gecko we must trust to keep competition and openness alive.