Yeah people always say "They could sell data".

I don't really see who would buy behavioral data on peoples browsing habits. That's fairly worthless unless you have a clear route to monetize it and connect back to those users.

GMail has a clear route - they show advertising on gmail.

All Opera could do is mess around with web pages, inserting their own advertising, or messing with content. Which would enrage users and website owners alike. As soon as they try to monetize, people will stop using it.

I use Opera mini extensively on my phone, probably more than the Phone/SMS aspects. I would certainly opt in for all kinds of intrusive behaviour (behavioral advertising, changing ads, adding appropriate ads sometimes) to allow Opera to continue offering that proxying service for free and allocating development resources when required. ie. My Samsung B3310 doesn't let me rotate Opera Mini so using the keyboard is awkward but surely there are so many phones out there that it would take considerable resources for Opera to continue to respond to all the feedback.

I use AdThwart on my desktop Google Chrome though!

I was using Opera Mini on my mobile under the assumption that they value my privacy and do not store any data. If they would start doing this, it would outrage me enough that I might potentially even go back to Firefox on my desktop.

The article also says Opera is aware that doing this could destroy their business, for the reasons you give, so they plan not to.