A number of customers (myself included) had 4
hour replacement contracts, which the vendor really could
not meet - so we agreed to take a new, much larger/better
model as a replacement.
It's one of the reasons we never pay for 24/7/365.
In many cases for our experience, with sufficient pre-built
redundancy, there isn't much different with 24/7 vs. NBD, apart from
the cost. So why pay for the premium.
And if a site does not require redundancy, it's cheaper to have a
cold standby than to give it 24/7/365, or even NBD.