There are always trade-offs
What the Office 365 e-mail outage shows is not an inherent vulnerability so much as the helplessness of clients that use these services.
I have clients on the cloud but for each and every one I have made it VERY clear that, while having someone else managing your e-mail might be seen as a benefit, there is a very real flipside, which is that they are the ONLY ones who can manage it.
It doesn't matter how urgent it is or how much you are willing to pay, if something goes wrong, you just have to sit there and let the cloud provider manage it. It does matter if you have to submit a $20m tender or are waiting to receive crucial information from an investor - it will take as long as it takes.
Without going into depth, when I setup an onsite mail system, I provide as much redundancy as the budget will allow and, with a very modest outlay, you can provide quite a lot of options.
The system won't be as reliable day-to-day - sometimes e-mails will bank up if their server has an issue - but the point is that almost any issue can be fixed and fixed relatively quickly.
As I've said many times, that doesn't make cloud services bad, there are just always trade-offs, just as there are with on-site systems. The important point is REALLY understanding those trade-offs and the potential risk to your business.