Windows is a paid-for product; either enterprise licenses or via the manufacturer. Yet few people (mainly those who build PCs themselves) realise this.

What if the browser had a similar model? The manufacturer pays a certain 'browser development fee' into escrow, then on first boot, the copmuter shows a browser ballot, which gets set as the default, and the fee goes to the chosen browser developer? There's probably a bunch of problems with this approach, and, at least initially, wouldn't break the monoculture, but it might be a good starting point for how to fund browser development.