About linearity, you have to be at a certain point sure, but it's not I have to be 6 to take on their 6's. Tactics matter. So lets say I can take on 7's and 8 baddies at level 6. Well now I have 8 levels that I can play against 1-8, why is that linear?
We also know the other side of this argument, it reached its pinnacle in Bethesda's Oblivion and it got lambasted where all encounters were scaled to you. Everything was the same fight, you could go anywhere without risk, etc etc.
There is no overall win on this one, we've been talking in circles as a gaming community for year. We can just hope whatever the designers make it will be well balanced and fun. Old school says I can't go anywhere I choose, I will hit a high level mob that forces me to go look elsewhere to gain strength. Otherwise no area is sacred, you can just go the straightest path to the end of the game.
The problems I frequently run into with the scenario you described is that moments of extreme challenge end up creating moments of extreme boredom. For example, let's say I'm level 5 and come across an area that has encounters ranging from level 4 through 8. "Freedom" allows me to take on the level 8 encounters first, which (while challenging) also quickly level my characters up. The drawback here is this freedom has now bitten me in the rear, as I now have content to deal with that is two or more levels below me. Content that will be, unfortunately, easy due to my level advantage (and easy to me = boring).
The fact that you felt you could go everywhere in Oblivion without risk is not a problem with level scaling, but rather Bethesda's interpretation of the concept.
I would happily welcome a system where I can never outlevel content, where all my opponents are artificially raised to my level (or at least a level within my level). At the same time, you don't have to decrease MOB levels to match the player, meaning there are still plenty of encounters out there for the player to be threatened (or killed) by, encounters the player can grow into. The key is to not let them outgrow the encounters, though.
At least, that's how I like it. I'm well aware that some people love to be able to get several levels over their opponents so they can feel some sense of power while one-shotting them, but for me that will always be wasted content that I prefer not to experience. I'm also aware that some players define "growth" and "progression" by being able to vastly out-level the opposition, but I am content with other forms of character growth.
I will always feel like I have to tackle Divinity : OS's content in a very particular order, lest I create a situation where I'm doomed to fight against trivial encounters due to an extreme level advantage on my part. And for a game that's supposed to embrace freedom, that really is a shame. I think it really may be the weakest point of the game for me.
