Progress and difficulty

I am reading a paper on personalized difficulty in games and I see this graph:

The graph shows clearly that the probability of playing tomorrow depends on how many levels (amount of progress) you beat today.

At GDC24 one of the talks showed this graph:

Kills in 10 matches of a FPS will influence the probability of playing again.

What can we learn from here?

• Theory of Flow is always valid
• It’s about the Player: the player of your game should feel good enough and progressing enough. It’s about them, not you!
• You need to deeply understand the elements of friction, goals and mechanics that shape up the difficulty of your game.