you can do that in basically every single Final Fantasy as well. And a lot of JRPG's, like Dragon Quest. Persona is different, as is TWEWY, as the games are day by day. 

really ???? I'm pretty sure you take out multiplayer out of the top selling FPS and the IP is dead or close to it.... multiplayer is one of the most important element to certain category of gaming now a day.... and still that was just one more layer you missed out.... what made halo a classic of XB is mostly there multiplayer same with blackops, and GoW....
then again to me with your def of linearity.... your non linear exemple are biased.... I found them as much linear if not more.... the real question at the end of the day is do you like the game you play or not.... not how much they are linear or not.... you have good ones on both side depending on what you expect and or like

So if the servers are gone for example the game isn't a classic anymore right?

if multiplayer is what made Halo so great then why do you think people demand it to be Remade? they should have just played Halo Reach and get on with it, think about it for a second, Multiplayer is never a big factor of why certain games are considered "Classic", do people remember Call Of Duty's campaign ? No , Do people remember Half Life 2's campaign ? Yes.

Linear maps: This is about exploration. Linear maps do have some exploration, although it's merely finding a startingpoint of a rollercoaster or looking in corners for some items (Uncharted). Fallout is randomly random you can go anymore you like. There is a lot to explore. GTA is also openworld, but it's pointless there is nothing to explore in their big citys only cars and pedestrians.

Linear enemy's: No matter how smart an enemy is, it all comes down to their spawnpoint. If you know where they are spawning it becomes a lot easier. Many games have fixed enemy's fallout, deus ex, cod, but there is one game I'm aware of and that is Left 4 Dead with random spawnpoints.

Because non-linearity the definition of a video game... the ability for YOU to control what happens.

Linear games are just shit movies, basically and I can't think of a SINGLE great linear game. Not one. Although I can see an argument that Half-Life or Mario (NSMBW) are linear, weak one but it exists.

Also, non-linearity doesn't mean a game is good.

Finally, I don't buy your Mass Effect linearity argument one bit. The rest are spot on.

That is false, because Thief II is a much better stealth game than Hitman ever was. Hitman is still pretty damn awesome though.

Burn the heretic!

Combat in Mass Effect is pretty linear in my experience. Spawnpoints are always the same. Different classes are cool, but you have to stick with your class through the whole game. In other words you don't have much options in the middle of the game. I didn't bother changing my upgrades, but I know that's a posibility with a penalty. Level design is also straight forward. 

Mass Effect is a rich game with multiple parts of gameplay. Ultimately I agree with you, want do you think of the combat in Mass Effect?

Oh god , what have you done! , that is one of the best games of all time.