Let's think about it for a second. Being on Steam, the game would sell more copies at first, then more than half of those who just bought it would probably down-vote it by saying things such as "graphic looks ps2, pcars is twice as cheap and got 10 times more cars" or "Need for Speed is better cuz you can upgrade your cars", "driving sux it's too hard" and that kind of stuffs. Imagine the legion of frustrated players from the last ten years that would bash the game on first sight because for some a racing sim is like a religion; you can only worship one at a time and hate the others. Then this unique gem of a sim with long term support and high replayability would end at the bottom of the Steam racing category because most people want eye candy when it comes to the racing genre and god knows the first impression decide whether or not a person love or hate a game. Those still interested would wait for a sale (surprise) and those who missed the sale would wait for it to go on sale again because they've seen it on sale before so they'd rather wait for another sale to get it cheaper therefor, the game at normal price stagnate. Steam sales are probably the main reason people wants to see LFS over there. Sales are great when your 2 years old product support comes at an end and you want to boost sales while you can, but this game doesn't have that approach so sales could hurt it very much in the long term.
Now the Workshop? Even Assetto Corsa stays away from it because most of the mods would be ripped-off from other games or build-from-scratch mods (95% low quality model and questionable driving physics) of prestigious license that hasn't been paid with the problems that follow.
I already have LFS on Steam. I just added it as a non-steam game and I can launch it from there (yay!..) and the Steam overlay works just like any other official game on Steam. Beside dividing the forum from here to there, I don't see any major points.
LFS is the only commercial sim in development (as far as I know) that hasn't been swallowed by Steam. It makes it unique and I would go as far as to say that some people might have bought it especially because it doesn't depend of a third-party game launcher DRM that makes you agree to all sorts of new EULA every few months.
If the thread would be to put LFS on GoG (where a third party launcher is not mandatory), that might be more interesting as it would be the only driving simulator over there so it would have much better visibility. If Scavier could circonvent the DRM-free policy by providing a DRM-Free LFS that would require a key only for online racing or something like that, I guess that would be a better spot than ending in the Steam jungle.
I love Steam, I use it several times a week. However I couldn't care less that LFS is on Steam or not. What I care about is when I log to play in multiplayer and get nothing but cruising servers. Then I race on an empty server and eventually get tired and leave. Steam could solve this in short terms, but it's not a miracle solution.
I'm not saying LFS isn't nice visually btw. But some track shaders and more life around the track would make a greater impression to newcomers before having it on Steam.
I'm ready to get flamed because I did not praise the Mighty Gaben