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Hosting shit yourself != more secure, and only someone with a highly naive view of their capabilities as an organization would make that assessment. Facebook is written in PHP too, but you don't see that being a huge secure vulnerability, do you?

Slack has an entire security organization dedicated exclusively to securing its stuff. My security team is focused on securing our operational systems.

Do you run your own bank? How could you outsource something so critical (literally all your money and financial details!) to a 3rd party who doesn't even let you audit their stuff?! It's just a matter of time before there's a huge incident.

Do you run your own electrical generation facility? How could you outsource something so critical to a 3rd party? I bet they don't even have an SLA!

etc. etc.