I have always felt there is something fascinating going on behind Flappy Birds' infamous difficulty curve that warrants deeper study.
On the one hand, there is no actual progression or ramping of difficulty in the game itself. The difficulty level remains the same whether your current score is 0 or 10 or 100. But every new highscore represents a new summit that the player has to scale. The first and maybe the most frustrating summit to scale is scoring a single point. To get your score into the double digits, the player has to have basic mastery of the core mechanics - including the precise physics, and timings- and learn how to handle a certain number of scenarios. The obstacles on the path to triple-digit territory and beyond seem almost self-imposed. The fear and tension as you approach your own highscore is the biggest impediment to breaking your highscore. Once you break that highscore - the hand tremors magically disappear the next time you approach it, only for it to re-appear as you near your new highscore.
All this, when the basic concept of the gameplay is deceptively simple. Like i said, there are many layers to unpack for someone who is willing to look into it.
Look at almost ANY arcade game of the 70s/80s. Most of them weren't designed with an end in mind. You play as long as you can for the accolades that come with posting your initials on the scoreboard.
Asteroids is a quintessential example - relatively flat difficulty curve once you've mastered the game - it really comes down to a test of the player's endurance. Scott Safran set a record game that lasted a grueling 60 hours.
EDIT: Anyone who has EVER tried for a high score (whether a personal best, or a world record) is familiar with the natural nervousness that increases in direct proportion to how close you are to breaking it. That's not a Flappy Bird thing, that's a literal every game thing. Go watch a live stream of a speed runner that's got a heart rate monitor attached to the feed for example.
Many early 80s 8-bit console games worked this way. I'm thinking of games like Transbot for the Sega Master System. There are a few different levels, with various enemy configurations and scenery, but it has no ending and goes on forever without any real changes.
This was the case for me with Flappy Bird and also that old Temple Run game circa 2011-ish.
It's almost more of a game of focus or how much you will _think_, because once you're distracted a bit and forget those physics or timing just one time, you're probably done.
On the one hand, there is no actual progression or ramping of difficulty in the game itself. The difficulty level remains the same whether your current score is 0 or 10 or 100. But every new highscore represents a new summit that the player has to scale. The first and maybe the most frustrating summit to scale is scoring a single point. To get your score into the double digits, the player has to have basic mastery of the core mechanics - including the precise physics, and timings- and learn how to handle a certain number of scenarios. The obstacles on the path to triple-digit territory and beyond seem almost self-imposed. The fear and tension as you approach your own highscore is the biggest impediment to breaking your highscore. Once you break that highscore - the hand tremors magically disappear the next time you approach it, only for it to re-appear as you near your new highscore.
All this, when the basic concept of the gameplay is deceptively simple. Like i said, there are many layers to unpack for someone who is willing to look into it.