Let's do the figures.
1) Let's count for simplicity that project valuation equals to 10 Annual Revenues. That means that the annual income should be $100M, and monthly income is ~$8M
2) $8M means 400'000 clients paying $20/month.
3) 400'000 paying customers means 20'000'000 total users. (since average conversion is about 2%).
4) There are 5.5 billion internet users worldwide. So 20M users are 0.35% of the total audience.
So, probably, it is achievable to find a specific niche product for these particular 0.35% of the users that is possible to build solo. AI-powered solutions fits even better to this concept.
