Re: But then comes downtime.
Do a Risk Analysis comparison between on-prem and cloud. Simplest example: A company with one pc. Extrapolate how you wish. All risks are "in series" ie., one link fails, everything fails.
On-prem risks: pc hardware failure, malware. (Hardware maintenance can be scheduled, so less of a risk).
Cloud risks: Outage at cloud provider, Administrative problems at Cloud (preventing login - is account-holder around to verify credentials?), telephone outage (impacting on broadband), broadband outage (in its own right), hacker issues (e.g., DDOS), hardware problem with router, problem with LAN cabling, pc hardware failure, malware.
EDIT: Some may say that Multiple Points of Presence dilute Risk. Yes, in some cases. But it all depends on whether your data has been successfully replicated to other points of presence. You may be accessing out of date data. This is where those who think that Cloud is a solution for non-techies need to review things... with their techies.