I think that we are at the point in the game that people don't want to pay for a browser unless they can get something significantly better than what they get from a free browser. Keep in mind that in breaking into the market, you have to overtake the competitors with features that aren't available. This would likely be possible in the mobile market, but not the desktop market.
Opera's market share has been both increasing and decreasing over the last few years[1] but it looks like it is taking market share from IE users and more people are switching to chrome. So I would predict Opera's share would go down in the future.
The big questions for me, would then be: What am I getting from a browser that I pay for over a free one? Can I get the same features out of plugins for a more widely adopted (and hence better supported or developed for) browser?
Microsoft is really business and student oriented with the majority of their products. A browser only matches part of their design goals. They want people to keep paying for their OS and don't want products to migrate to the web where they have to compete with Google (Though they already have to do this to some extent).
Don't get me wrong, I would be personally interested in something like this but don't believe that many people outside of developers (and people doing criminal things whether moral or not) would be.
Not sure what you mean with the last point/question. The market share would be decidedly small, it likely wouldn't be developed for (unless it used a major library like webkit) and most business people wouldn't notice much difference in their ad-based revenue from such a small market share.
[1] http://www.w3schools.com/browsers/browsers_stats.asp