If you book a hall for a wedding and the hall you are renting has a max capacity of 100 people buy you invite 400 instead - if something bad happens that makes you the responsible party for enabling those problems and people can hold you accountable for everything.
Just because a game can host 100 million players it doesn't mean that it should. If Roblox are unable to police their own game and ensure all players are able to enjoy the game without being constantly approached by bad actors they should simply limit the amount of people who play the game. If you can police 50 million but not 100 your max player number is 50 million. Simple as.

You can't just bait a bunch of kids into joining your game and pretend you are not responsible for anything bad that might happen.