There is some security even if they provide the public key. Bootstrapping is a problem, but clients can keep track of a mapping from package names to public key and issue a warning if that ever changes. That’s how SSH and RDP works and while I’ve never had an actual security hole plugged with this, I’ve had cases where my remote machine went down so the DNS didn’t update yet the IP was reassigned so the warning about mismatching keys was actually helpful.