The walkthroughs are great for niche games that don't have many players, true. But at the same time, it's not like Steam's walkthroughs are the only ones or best ones on the Internet. Any game that has enough interest for people to make walkthroughs for are big enough to have a fanbase that maintains a wiki, or a subreddit, or a discord, or some other place where you can exchange the same info. Hell, you could even stay logged into your Steam account in the browser and just look at guides that way, while playing the game itself in another window.

They're not useless, but if I were to think of the things that keep me using Steam as opposed to using a new launcher (or alternatively, the one thing a new launcher needs to get me to hop over from steam), walkthroughs aren't really high priority imo.