Actually I strongly disagree that it was necessary to stream down Opera because of its lack of user base, because its lack of user base was due to the lack of marketing to point out to users the differences and advantages of Opera 12.17 over its competitors. Had it been marketed well the user base would have certainly climbed, as practically everyone I would tell about Opera's useful features would be sold and converted. ...
@davidmichaelmorrell, this has all been hashed and re-hashed countless times over the past couple of years in Opera's forums. As I have come to understand it, Presto Opera was 'streamed down' because of the heavy cost to Opera ASA of maintaining and updating its own custom Presto rendering engine, along with the effort needed to obtain and retain browser compatibility with loads of popular websites which had no interest in coding their sites to assure Presto compatibility.
Once the decision to replace the Presto engine had been made by Opera, it was apparent the rest of the browser would have to be rebuilt from the new engine on up. Then the questions became what features should be coded into the new browser and why? The strategic decision was made by Opera to pursue a much more streamlined browser having much fewer user-settings, and to export responsibility for user customization to individual extensions for the users that wanted them. Users may disagree with Opera's conclusions (and many of us did at the time), but they were Opera's alone to draw.
As @donq has asked, just how much involvement have you had in software development at the major level of a web browser, all squarely in the public eye? Asking "just how difficult can it be?" betrays a lack of hard experience in how difficult things of this sort really can be, how long they take, and how much developmental money they end up costing. At the end of the day, Opera ASA still has to make money for its stockholders/owners, so the development costs rolled into a 'free' browser have to be carefully considered ahead of time.
I write all this as a 'niche' techno-user myself, one who really desires deep customizability in my browser, rich built-in feature sets, and so on. But I also realize that many of today's web users only want a streamlined browser with a number of unique browsing convenience features built in, rather than the ability to customize every jot and tittle of the browser interface. When I want and need those configurable elements, I can turn to Firefox (at least for a little while longer), Vivaldi, Olde Opera, or something like those - and I often do. But I don't expect Opera (or even Chrome) to abandon its chosen market and development strategy just to please me or some others like me. Each browser brand is whatever it is, and every user has to navigate among them in the marketplace to find what works best for that user. At the end of the day, the browser maker calls the shots on his browser design and which users he targets with it.