You kinda do, except you get ambushed and die. Because the base on Deimos is actually part of hell now, that's where you respawn, in hell. You clear out THAT base and it's new commander and actually go out to the outer levels of old school hell. And then again, you clear THAT out, and kill the guy leading the invasion on Mars. The guy leading Hell basically gives you a teleporter back to Earth so you'll stop messing stuff up. THAT'S RIGHT, HELL BROUGHT YOU BACK TO LIFE BECAUSE YOU KICKED IT'S ASS.
Then you teleport back to Earth, and OMG IT'S OVERRUN TOO. Apparently you meet the last few survivors and get them onto a ship so they can get out of Dodge. The problem is, base where the launch button is located? Overrun by demons.
You clear THOSE demons out, launch the ship and save humanity. Holy smokes, doomguy you're such a stone cold badass! Because you're so tired from destroying countless demons, you decide to lay down. Then some commander comes up to you and says "Hey thanks for saving humanity and stuff. We found out where the demons are coming from though, your hometown." So you get back up.
THEN you fight your way through your hometown and find where the demons are coming from. Do you go through the portal? Hell yeah you do.
There's more demons, so you fight your way through the SERIOUS part of hell, and then kill SATAN. Unless Plutonia/TnT is considered canon. Then it's just some bigass guy who is like Satan's best friend or something.
And that is why Doomguy is the single most powerful human in the history of video games. Even JRPG/WoW players need a whole bunch of people to take down a powerful monster. But apparently Doom guy did it by himself, without magic or friends.
Another fun fact about Doom guy:
His maximum run speed is 819.52 map units / second.
Approximately 32 is equal to one meter (based on Doom guy's height in terms of map units.)
Therefore he runs about 25.61 meters per second, that is equal to about 57.2879 MPH.
The current record for the 100 meter dash is 9.69 seconds and held by Usain Bolt. The Doomguy can run this in about 3.905 seconds, nearly 148% than Usain Bolt.
The DOOMGUY is capable of outrunning a Lion, and maintaining that speed. Keep in mind that he can do this in all probability, 200 pounds of equipment.
DOOMGUY is the strongest video game character ever.