1. an outage, which in reality is just an inconvenience, not the end of the world, unlike what most IT people seem to think.

2. a bill that can bankrupt you, which may as well be the end of the world for many people or small businesses. It can be literally "game over".

A bare metal box doesn't need protection from the 2nd risk. Its costs are fixed, irrespective of the amount of traffic attempting to hit it. A 100 Mbps link can't put out more than 100 Mbps, so even if you're charged by the terabyte of egress, there's a cost ceiling integrated into the hardware itself.

The cloud generally has no such limits, or much, much higher ones than is typically desirable.

Okay, here's another random example your WAF will not protect you from: cloud-hosted DNS.

The bare metal scenario is a box sitting on the end of a 1 Gbps Ethernet link. If attacked by some crazy UDP DNS flood attack, it could probably saturate that pipe and send out... 1 Gbps. On a fixed-cost-link plan this costs $0.00 additional money. You might have an outage, or merely a brown-out, but you won't see a cent added to your next bill.

On Azure's DNS Zones service, there's no "1 Gbps" pipe to rate limit them. They have infrastructure deployed globally, typically with 100 Gbps links. In practice, the DNS server probably only gets about 10 Gbps per region, but there's many regions. At 100 bytes per packet, you could be looking at a billion requests per second billed to your account, at an eyewatering $200/s or $720K/hour. Ouch!

Now, Azure will probably forgive that bill because it's clearly an attack.

But what if it isn't clearly an attack? Application Insights by design puts the Instrumentation Key into client-side JavaScript. It charges $3/GB on ingress! It's trivial to charge someone thousands or tens of thousands of dollars before they notice, and then they'd have a hard time convincing support that the traffic wasn't legitimate.

I can send a terabyte out for cents, each of which would cost some poor fool $3,000.

Good luck plugging every such hole, monitoring every alert (there's literally tens of thousands of metrics to alert on), and keeping up with every spike in billing that's a day late reporting on costs that can ramp up to thousands of dollars per minute.