I'm sure there are folks who are willing to pay that kind of money for this experience, I'm just not one of them.
I hope those folks are aware that Vivaldi gives them the same experience for free.
I believe Windows Pro license is a one time fee, so SigmaOS is more expensive than Windows
Likewise, there's nothing inherent to browsers that mean they always need to be free, just because the dominant ones currently are.
Now, that said, I don't think this product is worth anywhere near $15/mo to me, but maybe that'll change over time. A browser that actually makes me significantly faster at my work vs chrome/etc could easily be worth that much to me.
$10/month for Spotify because it's easier than maintaining my own music library and syncing it across devices.
$10/month for Lightroom and Photoshop because it's vastly better than its competition
I just don't see how this is /that/ much better than the competition (Firefox + extensions) or makes my life easier in any significant way.
At current prices, for $15/month you need to provide some serious value because most people will do the direct comparison to other subscriptions they are familiar with.
Make it a one-time payment and I'd think about it. Subware is not something I support.