We did that monthly subscription model: https://news.ycombinator.com/item?id=19038820

It had too few conversions and many of those were too small (< $5) to make it viable.

Ads are already effortless and fair, they generate cash from your attention in real-time and on-demand. The problem is the overall experience (interruptive formats, high frequency, tracking, etc) which is what Brave is really improving on. Good luck to them.