I respect your opinion, because even if an application could handle 200k users at once was able to be run on a MBP without serious issue, it's still gauche to forcibly invite 200k people in your company to a chat.
Slack is designed for business chat, and many businesses have thousands or more employees. Slack should be able to handle any features it provides, and @everyone is a feature. I don't know how they've implemented things or what the specific issue is, but it should at least have blocks in place that prevent stupid things. Maybe channel sizes should be limited to 1000 people or whatever their testing shows it can handle. Idk.
Software designed for large numbers of people shouldn't go down because one of those people does something stupid.
Software designed for large numbers of people shouldn't go down because one of those people does something stupid.