The major lesson is "don't make indie games if you want to make money". There are 34 games released per day on steam. Most will have revenue under $1000 (and probably most will have revenue below $100).
Unless you have a talented team, marketing plan, and lots of capital you can burn for a gamble; or are willing to just do it for fun for years and don't care if it succeeds -- don't quit your job to make indie games. Otherwise you're playing Russian roulette. For every hollow knight, Dwarf Fortress, Celeste, there are 1000s of other titles you've never heard of.
To be fair, you'd have to somehow account for what percentage of those released each day are just shovelware crap. The actual "indie games" to look at would be the ones where talented people put time and effort into something. A vast majority of those will still fail, but then you have at least a true basis.
Unless you have a talented team, marketing plan, and lots of capital you can burn for a gamble; or are willing to just do it for fun for years and don't care if it succeeds -- don't quit your job to make indie games. Otherwise you're playing Russian roulette. For every hollow knight, Dwarf Fortress, Celeste, there are 1000s of other titles you've never heard of.