Obviously we can only look at games that have achievements and include an achievement for beating the final boss/reaching the end.
I revently completed Crash Bandicoot 4 on steam and was very surprised to see that only 15.1% of the game’s owners on steam have beaten the final boss (there’s also achievements for 100% and 106% completions): https://steamcommunity.com/stats/1378990/achievements
The game is quite difficult, especially the last stages, but the number was still very low. For comparison Dark Souls 3’s most common ending has 24.9% for the achieved percentage. https://steamcommunity.com/stats/374320/achievements
Are there any other examples? Obviously games that have been bundled might distort the numbers because I would think that the urge to complete games decreases if you’ve paid very little for the specific game as it came in a bundle vs buying the specific game.