Valve released HL2 along with Steam. I'm convinced they'll release HL3 with their own, Valve-branded console. I'm also guessing that it will be an exclusive to SteamOS for 6 months or something.
HL2 launched Steam, HL3 would launch SteamOS.
Oh well.
Steam came out 1 year 2 months before Half Life 2.
They didn't really 'bribe' people, it was more to do with 1.6 CS only working on steam and patching was done via steam.
That's a pretty good "carrot" to use Steam if you ask me.
I only had Half Life, and Counter Strike was free when I started playing so I only got HL / CS. I didn't get the others free until HL2 came out...
https://steamdb.info/sub/1/apps/
I can confirm it worked with HL1, CS Retail and Blue Shift keys.