Re: Use a public provider?
I'm not advocating putting everything on public cloud but some of your points are relatively easily answered:
1) The public cloud vendors, or the big three at least, are now fully aware of how important data security is with ironclad contracts that they won't look at your data and enough encryption, sharding, sharks with laser beams etc. to protect it from outside spying.
2) Unless all you users are in the same building as your servers then you're already reliant on a functioning network provided by an external provider.
3) Unless you're a seriously profitable organisation or public sector then there's more chance of your business going bust than some of the public cloud vendors.
Public cloud can't directly replace on-premise IT but if you take it up a level does it have to? If you look at it from a real fundamental - what services do I need to run the business standpoint - you can find the real gaps and assess the risks and then it's up to the business to decide if those risks are worth running.