Super Mario Bros Wonder threads this needle expertly, in my opinion.

Each level has a difficulty rating from 1 to 5 stars with 1 being easiest and 5 being "Mario Hard."

To complete the main story, you only really need to beat mostly the easier levels, like difficulty 1-3 stars. All other levels are really optional, but there are a lot of them, and they are the 4-5 stars level difficulty.

So the "main game" by default should be "easy enough" for most gamers, and for those who want a challenge, there are tons of extra challenges for them to pursue.

I think I prefer this to a "difficulty setting" because it allows both casual and hardcore gamers to approach the same game in different ways. It doesn't make you feel like you are missing anything from either way you choose to play. It also allows you to practice the harder levels if you want to get better.

Some games like Halo, if I recall correctly, literally rewarded you with special cutscenes for the hardest difficulty in beating the game. That can leave players who "aren't good enough" for such high difficulty to feel a bit left out.

I don't feel the same about Super Mario Bros Wonder, it just feels pretty accessible to all and I think more companies need to attempt something similar.