Great, another online only game.
Like dota 1, you could always play offline 5v5 as the only human among bots.
Oh i guess there was also LAN in wciii tft.
it's sad as a huge Portal fan, but at this point Valve's been a platform holder longer than they have been a "console games" studio. The only stipulation is that they are free enough with their work that you have a 1% chance of turning your own Portal/half-Life Mod into a sanctioned release if you impress them enough.
I'd say "be the change you want to be", but I think the worst use of your talent is investing in an IP you do not own/have a license for. Every Garry's Mod has hundreds of broken dreams underneath. And I especially hate this line, but: If you're gonna mod you need to do it as a hobby. Because monetizing it is the easiest way to get on 99% of companies' radar.
Then again, I could have just said offline games, and maybe I was overthinking it.
If you liked Portal, give the Talos Principle 1 and 2 a try.
If you like story driven shooters, try Horizons, Control, Atomic Heart, Titanfall, Doom Eternal, the System Shock remake, Metro, Wolfenstein, Dishonored, Prey. There are more good ones from last decade too (FEAR, Rainbow Six, etc.). None of them are really Half Life replacements, but they're good.
Or if you don't mind open world, RDR2, Cyberpunk, etc.
Not to mention the dozens of amazing single player indie games (not shooters) that have sprung up on Steam over the past two decades.
Valve seems to have given up on single-player games, but their marketplace allowed the industry to really blossom into the huge thing it is today.