Completely agree about Conan Exiles. Great game, and the epitome of what an open world game is supposed to be. I've personally never played the Witcher games. Though from what I hear about them, I just might.
I have to wonder WHY this game feels open world to you after playing CE. Could it be that you feel this way because the world "seems" to open up once you complete "The Heist"? Because here again, is just another illusion. Because certain content is locked away for the sake of the main quest's story. Seriously. Go try to meet Panam right after the Johnny sequence in V's apartment. You can't. Now ask yourself why.
It's because she's not there UNTIL you talk to Takemura in Tom's Diner, which opens up the branches to find the guy who created the biochip, and the branch to find Evelyn. AND because Johnny has a line in each of the initial sequences with those characters. The entirety of the remaining game continues on in this SAME trend of illusion until you get to the point where you fork off into whatever ending sequence you've been working toward at Johnny's grave site. Why?
Because you're a hamster on a wheel and NOT playing in a truly open world game. And they put you on that wheel in..... yep, Watson. A game's "prologue" being a closed experience is not what I have an issue with. They're typical of most games as part of the "tutorial sequence" to familiarize you with the game, it's premise, and it's mechanics. Like the prison in Oblivion. Or the cart ride in Skyrim. Or Vault 111 in Fallout 4. In ALL of those games, once you're out of the prologue, you do what you want. Because that's the freedom of choice open world games give you.
My issue is that if you claim your game is "open world", then I'm going to expect that you built it as one. But they simply didn't.
I guess for me I break things up into three categories -- Non Linear, Open World, Sand Box -- And these are only my definitions...
Non Linear refers to how I play the narrative, and how many choices I have that can effect my play though of the game. I feel games like VM:B are non-linear, but not open world, or sand box.
Open World refers to games in which I feel that I am at a theme park, and I can go to any ride I want at any time, and I have the whole theme park open to me. In this game once they open the theme park, like you said after Takemura, I can do gigs, main story, or side quest. And I have my choice of when I can do it. Many use theme parks to also include linearity like in MMOs, not me.
Sand Box to me is when there is no prescribed path to follow in how I even play or advance the game. The game is what I make of it, to use Conan Exiles as an example, I can build stuff, progress through chapters, wander endlessely, ignore the story completely, it is completely what I make of it. And personally allows for the best roleplaying experience I have had in ages.
Furthermore, I feel like games can have emphasis in 1 or all 3. However, the more you do in all categories the more expensive a game becomes to make. Some examples of how I rate games.
Black Desert Online / Conan Exiles: Open World, and Sand box, but very linear
Witcher 3 / C77: Open World and fairly linear, with no sandbox elements.
Witcher 1/2: Fairly non-linear, not really open world, and not sandbox.
Daggerfall/Morrowind: Fairly linear, but both have elements of sandbox and open world
Arcanum: Non-Linear, kind of open world, and not very sandbox.
I have seen people mix and match these terms, but I have found that by breaking things up it's easier for me to categorize games. For instance people will describe MMO's as either sandbox or themepark, but Black Desert Online proves you can do both.