I'm seeing a lot of responses suggesting half measures or compromises and I'd like to throw in my two cents:
This is the last update, so most everything should exist in such a state that if this game were to be dug up 20 years from now, there would be as few hitches as possible.
Therefore, I completely agree with Master Mode being unlocked by default and I think it is misguided to bar it behind an unlock. Imagine if you lost your steam account and then one day you were able to play Terraria again. you want to relive the thrill of master mode that you enjoyed back when you were a teen, except the realization hits that you need to complete the entire game on Expert before you can play your preferred game mode. It's pretty dumb.
And yes, you
could just download BW, equip yourself with endgame gear, blitz through to moonlord and unlock it quickly, but:
1.) any time someone suggests "just go to builders workshop to get the items you need" it hurts because it points to a critical failure of a game system. In a perfect world, the game should give you everything you need to play it correctly without feeling the need to cheat, and for someone to bypass the feature in such a trivial way puts into question the decision to make it an unlock in the first place, not to mention PC-console parity. I don't know if there is a world with all the items on console but i would bet there's a server at least.
2.) It is still inconvenient, as it requires a download and more than a few minutes
Ever wonder why fighting games don't really have you unlock characters anymore? It was super annoying for tournament organizers to go through and unlock all the characters just so that pros could pick who they want. I think Terraria would be taking a huge step back if master mode was gated behind an expert moonlord defeat. Having a cool reward for beating moon lord on expert (beyond in game items) is a great idea, but making that a whole game mode is not it. Maybe you could unlock songs in the menu under an "extras" tab or something, that kind of stuff is fine.
And yes, onboarding new players is an issue, the option should be there, but with more warning. In addition, I think there should be an option (or maybe the guide could do this) to toggle down (but not up) the difficulty of your world. Master-> Expert-> Normal->Creative? (not sure about that last one). this would allow new players who were overconfident and on the verge of rage quitting still lower the difficulty without losing the progress they made, while preventing difficulty toggling cheese. If that can't be done, then adding the feature in the world selection menu to "copy world to new difficulty" would allow you to keep your old difficulty world and play at a more casual level in the other