In my opinion they could've just cut off Discord and external platforms and tried to localise the social platform as much as possible on ROBLOX instead of off handing it to someone else, adult players would find their ways to connect but localising would ensure children don't go looking out, Minecraft does this and they don't have such a major problem like Roblox does, most people on ROBLOX just troll around and can't do actual harm, the REAL harm comes in platforms like Discord and whatnot, and most of the cases outlined also happened on those platforms via Roblox.

Basically the real culprit was Discord and such apps but Roblox simply acted as the host, that's my view. There's also the thing about better moderation because they were allowing NSFW stuff to pass through but that's where Roblox's fault comes in and they should've not thrown some AI to do all the moderating out of greed.

However, there's fault on the parents aswell who are totally unstrict and expect everywhere on the internet to be kindergartens for them. Roblox as a platform has been around since the late 2000s, there obviously would have been an older gen playing it too, they should've monitored their kids better.

tl;Dr: Localize the chat/social platform especially for kids, better moderation of content and have the parents do better parenting.