Imaro
Legend
First, suddenly is poor word choice. Its not suddenly for the players at all. The players are not aware of any kind of "sudden" adjustment. They heard rumors that the caves were full of goblins, they go check it out, and find it is full of goblins. This remains true, no story elements change at all, whether those goblins are lvl 1, lvl 4 or lvl 8, or a healthy mix of many different types.
Second, I've repeatedly said "not everything". Range of challenge is informed by the nature of the locale. A cave filled with goblins has a lot of possible ranges, but still an appropriate limit. A powerful monster isn't going to level adjust to low level PCs. In a sandbox, freedom is ALWAYS an illusion (yes, in yours too). You can't freely explore something that isn't there. If the DM doesn't place it in the box, its not an option. If the DM hasn't placed a marauding dragon in the setting, never had villages attacked or even a rumor of such a powerful foe nearby, then the players suddenly deciding they want to find and kill one is not something the setting can reasonably do. Unless, that is, you are willing to adjust your setting for the PCs. If you aren't, then freedom is certainly an illusion. If you are, then, well, what are we really arguing about?
I like my, and my players, freedom. I run sandbox games specifically so the PCs can find the game they want to play and when they do, it will be challenging. If the PCs want to do something that I haven't accounted for in the sandbox at all, I like to have the freedom to work that into the setting, even though it wasn't there. I like to have the freedom to challenge them appropriately when they decide to spend their time and resources on something. The freedom in such a setting is always an illusion, the game is an illusion itself, that's the nature of the beast.
If I haven't placed a marauding Dragon in my setting my PC's are welcome to go look for one but they won't find it. Freedom doesn't mean having everything you want given to you it's making choices from your options that have meaning and consequences in the game. Perhaps if I roll a dragon on the wandering monsters chart, then one is sighted and appears but if not oh, well people spent their lives searchingfor the fountain of youth and never found it...doesn't mean the journey won't be interesting.
All I'm saying is that my players have enjoyed the feeling of having to run from an enemy, only to return for that enemy when they've gained in power and pay him back. If I adjust the challenges this is way less likely to happen, and this is just one example of why I like to set the levels at different points and keep them there... besides after 20/30 levels of always running into something we can clober well it starts to feel contrived.