There are any number of ways you can approach this. One is security. I know that a large number of private on-site business networks are NOT in a secure environment. In fact, many of them are simply installed in small rooms, closets and so on - with access from every employee and even cleaning personnel. I've seen where a UPS delivery person dislodged a T1 connection to where it brought down the entire company.
Then you have the possibility of natural disasters like fires, floods, tornadoes and so on. Would it help to point out what's possible in a data center versus the infrastructure they have in place onsite?
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