Opera already has a database
Opera has a database in-built for its mail storage too, it's just that it's disabled by default and the older 'files and folders' storage systems are used. There's nothing stopping any user from going into opera:config and changing it.
Database servers are clearly going to be part of all proper browsers in the future. As the Web evolves from a simple document platform to an application platform, databases are goin tp be required to speed things up. No, it won't be as fast as Opera 3 loading the original BBc News site, but that's no longer the age we live in, and in the same way that modern cars are hideously over-complicated in order to function *like* modern cars, so too must browsers get bloated.
Ultimately the proof is this: Firefox, Chrome and Safari are open source. If the market doesn't want the bloat then someone will download the source and release a bare-bones browser version of one or more of those apps. People were motivated enough to release a version of Chrome without Google's spying, yet there is no version of Firefox or Chrome without the bloat. Sorry if you don't want to hear this, but that means only one thing - No one wants a bare-bones browser, they actually like or at least don't mind the bloat because they have quad-core processors and 4GB of RAM loading apps from a SATA drive, and it all runs perfectly well for them.