It works just fine. No reason you couldn't adjust your billing cap on the fly. I work in a medium size org that's part of a large one, and we have to funnel any significant resource requests (e.g. for more EKS nodes) through our SRE teams anyway to approve.
Actual spikey traffic that you can't plan for or react to is something I've never heard of, and believe is a marketing myth. If you find yourself actually trying to suddenly add a lot of capacity, you also learn that the elasticity itself is a myth; the provisioning attempt will fail. Or e.g. lambda will hit its scaling rate limit way before a single minimally-sized fargate container would cap out.
If you don't mind the risk, you could also just not set a billing limit.
The actual reason to use clouds is for things like security/compliance controls.
