It sounds like both of you are talking about cloud stuff when thinking about datacenters. I can see how that many layers and cross-checks would be necessary when all you really control is running memory and some pieces of storage, but a lot of that is due to the platform. When you control the actual metal, third-party monitoring services are much less necessary.

For a real DC, when it goes down I get a phone call from a human. I don't have to reinvent that process. If it's my own server room, I use a landline and a modem for OOB "dude you gotta come down here" notifications, a WAV of Woody Woodpecker or something. If the phone lines are down, I look at the newspaper headlines to see what happened.

There's no reason not to set up a standalone monitoring regime. Whether or not you use heartbeat notifications to tell you all is well is a matter of taste, but there is definitely more to maintaining your nines on a daily basis than simply adding more layers of monitoring.