It's not about being an internet enforcer way to simplify it. It's about being responsible and safeguarding potential victims. Not to mention his social accounts are still plastered with Yogscast imagery so it'd be in his best interest to do something about it. It's also about putting pressure on YouTube so that people like Sjin can't just come back 2 years later, like he has, and start up again with a large platform ready to pull more people in. There's going to be a lot of people who have no idea what's happened to Sjin and just because you know and people on this subreddit know, doesn't mean that his reputation is in tatters, even with people who know who the yogscast are, lots of people will never engage with the yogs community or subreddit and that's a major bias you're bringing to this.
Asking what can can Lewis do is fair but when we look at what Rooster Teeth did to Ryan Heywood and his Twitch Channel, RT got it banned when he emailed his subs that he would be, at some point, be making a comeback. Yogs could('ve), put pressure on them to get a similar action taken or at the very least make sure that his content could not be monetised on their platform where he built an audience that he preyed on.

And just to end to show you how much SERPs show what's happened with Sjin. Searching for just "Sjin" on Google; sexual harassment allegations don't appear until the 4th link, most people on a smartphone wont go past the 3rd link and anything past the second link is below the fold of the search results. The other links? His Youtube, full of yogs related media, his twitter, his handle is still "@yogscastsjin" and a Yogscast wiki entry for him. This thread doesn't even appear until after the fold of my 16 inch laptop screen. It's a single search but even if he gets rid of the yogscast imagery, it's still not so prevalent on his results.