While I'm not absolute on the notion that 5 letters is perfect, I found the current game lives on quite a precarious balance; There is a Japanese version with around 65 possible letters, and the secret word being 4 letters, but this turns out to be significantly harder, with the game rules adjusted accordingly to allow up to 12 tries. Despite being able to play it in my mother tongue, I found it less fun. Tweaking the formula (even in English) will likely require lot of thought into re-balancing.

Ok I’m ignorant. Why only 5 letters?

Also one could argue the root cause of those many, complex reasons is under-investment in R&D based in the assumption “the current product is perfect”

Timing for the game loop and accessibility.

5 letters gives enough options without requiring the player to have access to a lot of guesses to have any hope of winning, and gives the game a short playtime.

It's built to be quick to play. You can play it on a commute, or dip in a few times throughout the day.

You wouldn't have that progress visible to players with a larger problem space. Tangentially, you'd also wind up having to lean more on "technical" (as in scoped to a particular domain, not technological) language in order to fill the list. That limits access to those outside of that field of study.

You’re just describing positive aspects of the game in its current state, not “anything other than 5 letters causes the game to fall apart”