So... angel investing was once a very small and rare thing. An investment class well outside the scope of formal financing channels. The "angel" was a relative of "silent partners," the informal early financiers of small businesses.
We can call YC the "turning point," nominally. Now we have an extremely mature web of financing channels. People with degrees specializing in the sector. Formal pipelines. Scale.
The key point is that these were investments "no one else is willing to make." Hence "angel." Mike Markkula wasn't competing for his deal with Apple. That changes everything.
Most startup investments made today would have happened whether or not any specific investor decides to do it. The market has liquidity.
Useful framing. Then follows that when supply of capital goes up, returns go down. i.e. we don't need to believe that the world is conspiring against angels, but in the efficiency of markets, and therefore earning an acceptable return is (much) harder.