I think we just got older. People are still having a blast online, but they’re mostly kids or teenagers. Garrys Mod got replaced by Roblox, forums got replaced by Discord, blogs got replaced by vlogs. There’s still lots of fun and community still to be found online, people wouldn’t spend so much time on it if there wasn’t.
I don’t think it’s accurate to say Roblox replaced Gary’s Mod. They both released in 2006, and they’ve always targeted different audiences. Roblox has always targeted younger kids while Gary’s Mod has targeted older kids, teenagers, and adults. Roblox has had a weird resurgence in the past few years, but it didn’t replace Gary’s Mod. Heck, I’d reckon it was always more popular than Gary’s Mod among younger kids (I kn…
Maybe a better comparison might be Source engine custom games to Minecraft mods?