Thanks for adding that caveat; it's an unfair comparison IMO to compare the well-selling games of 25 years ago with AAA games of today. Half-Life was built by a team of about 80 people according to a quick Google, and there wasn't the ecosystem of tooling, resources and outsourcing that we have today, versus hundreds if not thousands of people (if you include outsourcing / engines / etc) for AAA titles today. And the modern day game devs will have enjoyed a relevant education, whereas back then those educations didn't exist yet. Notably, John Carmack did a lot of ground work in translating math into 3D video game engines; he was behind Wolfenstein, then Doom, then Quake, and the Quake engine was used as the basis for Half-Life's.

Anyway, 80 people is pretty substantial even at the time; for comparison, indie hits like Hades had ~20 people working on it, Hollow Knight's Team Cherry has just 3 employees (but they used Unity so they didn't have to do much engine programming, and the ports to various consoles was outsourced); Wube (Factorio) has had a few dozen people working on it. "indie" hit Kingdom Come: Deliverance had like 240 people working on it.