If you look at certain aspects of HL series, it probably is mediocre. But like a lot of other good games, it's more than the sum of its parts. All the elements play together so well. This is really important: For example, the Gravity Gun amazed me much more than BioShock's telekinesis power, despite them having a similar outcome (and I played BioShock first, so it can't be the novelty factor). That's probably because of how it was presented; Alyx and Mossman talking about it, then the walk to the junkjard that worked as a build-up, and then immediately getting to play with it and explore its possibilities. It's all about the inclusion of the particular elements into one world that feels self-contained, exciting and explorable. I could give more examples for that, but you probably know what I mean.

To get back to your question: Yes, HL3 really is still relevant imho. The problem I see with a lot of newer large-scale games is that they don't have this idea of a self-contained game world. Instead, you are given a pretty nice scenario, an OK plot for that scenario, and an "old box of toys" (got that accurate description from a zero punctuation video, don't remember which one). This box of toys consists of gameplay elements that are supposed to be entertaining and make the game look original, but still be a safe bet (wow, fish AI! Unnecessary boss battles! Tacked on RPG elements!). While the story is often subordinated and told in cutscenes where control is taken away from you. HL2's toy however -HL1 didn't really have a particular one-, Physics, gets specifically implemented into the whole thing, by presenting you the gravgun that was build by a fellow scientist, and is crucial for winning the endgame in the citadel. My point is, Half-Life 3 could finally be a return to HL's style of quality worldbuilding that actually matches with quality gameplay. Some good, newer games already were able to do that, but just very rarely in HL scale.

I also doubt there would be a big problem with HL3's appeal to gamers. Lately, I've been able to show HL2 to two different kinds of gamers (one is your typical AAA fanboy, the other one more of an "art gamer"), and they were both amazed and not bothered by linearity. Valve is just very good at hiding this, and Episode 2 already deflects from the corridor shooter scheme. Think of the awesome fight at the ending, were you drive through this forest and hunt striders. That's why I'm also pretty sure HL3 wouldn't be as linear as the first HL games, thus be even more attractive to a gaming society that is used to open-worldish games.