I saw this app youlearn.ai on YC and thought of building it. They are claiming to have 1M+ users with just one feature which is to convert your learning material into summaries, quizzes etc.
I gave this a shot and cloned almost most of the features in just 9+ hours of continous vibe coding (with 20% manual though).
Now I'm selling this on mvp marketplace. Might probably open source it in future or give it free.
That means what you have really proven here is that distribution and polishing are the crucial ingredients for a successful startup.