I am aware of this philosophy related to restricted content, but it seems to be inconsistently applied to certain aspects of the game. For example, once you unlock the level 30 toad arc, you are free to complete that when you wish. However, players opt into doing it earlier in order to reap the rewards (exp) as soon as possible. And even after completing this arc there are other reasons for players to return to the land of toads, like farming raccoons for a certain cosmetic or using it as a peaceful place to meet rivals. Same for the land of iron, which will probably see a lot more traffic once the level cap raises one day.
I believe this type of content works fine for certain smaller bosses or other instances. Spider boss is a very good example that I think fits with what you are trying to achieve, similar to the dull edges and sharp minds mission in Takumi. Personally, I view the Snake lair somewhere in between something like the cursed host cave bosses (no level restrictions at all) and something on the scale of Toad or Iron.
I genuinely think the snake lair is one of the coolest parts of the game, solving the maze and eventually finding a boss. It's just that me and many other players feel as though it could be put to better use, especially when you look at the content of the rest of the game.