I agree that its probably not going to impact the experience much if you haven't played the previous games, but your comparison to Doom makes no sense.
The original Doom games intentionally had almost no story. The new Doom was a complete reboot so even if the old ones had a story it wouldn't matter at all. HL:Alyx is a continuation of the story that falls between 1 and 2.
Agreed. These games are designed so you can start with any of them and still have a good time and be able to follow the story, just like any major movie action movie series. Sure, you might not follow quite everything if you haven't played/seen what came before, but you'll follow enough so it really won't be a problem.
Getting dropped in medias res without knowing 100% of what is going on is just an ancient trope of story-telling.
It's certainly a trope. But is it ancient? I'm trying to think of ancient examples of this but I can't. Are you sure it's not a modern trope?
I think Homer's Odyssey is ancient enough.
Doesn't really count when it's a sequel.
Why oh why would you need to play prequels to enjoy a game?
Half-Life is a series where you have guns and you kill shit in an inherently interesting setting: earth invasion. Getting dropped in medias res without knowing 100% of what is going on is just an ancient trope of story-telling.
Look how much people enjoy the latest Doom game. You don't need to have played the original to grasp the idea of demons pouring into earth through some sort of interdimensional gate. You aim and you shoot them.