No one denies that capacity planning is hard. There are books written on the subject. The points you make are exactly the reason why you need to do capacity planning and plan for mitigating failures. If you aren't planning on 2x (in fact more) growth then I'm confused as to what kind of growth you really expect in your service.
If you aren't giving yourself room for expected and unexpected loads, you're doing it wrong. Add capacity and load testing to your process.