An open world game should still have a sense of challenge and...well, actual level design.
Not saying BotW lacks these things because it doesn't. But I don't like the argument people use that just because it's an open world game, it shouldn't have any restraints on anything whatsoever. Open world means just that - it's a large world that you explore and it loads seamlessly, so you don't deal with immersion breaking loading screens for every area.
But that doesn't mean there can't be contextual restrictions on certain things. And BotW does this at many points. Divine Beasts, for example, can't be boarded before you do the quest to "calm" them. Attempt to do so, and you get blasted back by a force field. And even if you manage to glitch through said force field, there's a kill plane that automatically kills you if you land on the Divine Beasts before doing the quests you're supposed to. And then there of course is the fact you can't get off the Great Plateau without running some errands first (which also has a fail safe in place even if you use some very obvious methods that would otherwise allow you to get down safely without a para-sail, like rock launching with Stasis).
So it seems a bit odd to me that there is absolutely no challenge or contextual restrictions regarding actually getting to Ganon. I don't take issue with being able to go straight to Hyrule Castle, I take issue with the fact that Ganon's door is literally wide open. Like, the rest of the castle is full of enemies and malice that obstruct you from finding some secret areas and what not, but there is nothing of the sort where Ganon is himself. So I think they simply should have included something you had to do within Hyrule Castle itself before you could get to Ganon, to give players reason to explore what is arguably one of the most lovingly and intelligently crafted areas in the entire game.