I don't see a point in putting entire game data on Steam if it is not about to be updated through Steam. Imagine two possible scenerios with game installation:
1. You start the download and go to sleep. You set up your alarm clock for 1:00 AM as you already know you'll need to update it before you can play. You wake up, launch the game. It starts installing prerequisites and you can't wait because you are half asleep. Once the launcher actually starts updating, you are fully woken up and you can't sleep for the rest of the night. You are not well-slept and make a big mistake in your work at NASA, directing the rocket to White House instead of an asteroid, called Naglfar, getting dangerously close to the Earth. You get death penalty for terrorism. No gaming company will get a single payment from you anymore. Also, 4 weeks later the asteroid hits Earth and no gaming companies will exist anymore.
2. You start the download from Steam. You wait for the launcher to get downloaded, then launch it, start game download and go to sleep. When you wake up, the game is installed and ready to play. You start playing. After 10 minutes you get phone from your boss that you are getting an off day today, because the asteroid you were about to shoot has changed its direction, as Loki noticed that there are still good people on the Earth and it is not yet the time to destroy it. You can play all day. This is your first time you are happy since 20 years ago. Happiness also affects your sexual ability. You spend all night with your spouse. Nine months after, three potential Zenimax Online customers are born.
What's worse: imagine you have reinstalled Windows and want to restore the game folder from backup.
1. The files have finally finished copying, so you click Install in Steam. It verifies the files and starts to re-download half of them because they have been changed by the launcher. It is a lot, so you go to sleep. When the alarm clock rings at 1:00 AM, you launch the launcher and start to update. You go back to sleep. When you wake up in the morning, you notice that the patching has failed, so you click Repair in the launcher. You start eating breakfast and dressing up for work. Then you return to the computer only to find out that the game has been verified and no errors were found. You think "Maybe there was an update and Steam just downloaded it" and start the game. It freezes for 5 minutes, then crashes. You strt it and the same happens again. You spend another 10 minutes looking for a solution on Google. Finally you decide to uninstall it and reinstall from Steam. You are late for work. And you know you won't be able to play right after returning - you'll have to update the game. Finally at work, full of stress, you launch all rockets you have instead of just one. As a result, the jet force of the rocket launchers push the Earth away from its orbit around the Sun. Great winter without sunlight comes to Earth. Every single human dies to hypothermia. Except one man and one woman that were in space when the rockets got fired. After three years the Earth hits another planet called Asgard, finally ceasing the existence of both worlds.
2. You do that copy-paste job and play straight after... What's better?
TL;DR: Only the launcher should be downloadable from Steam.