The word you should have focused on was 'magic'. In the same way that magic does not exist, it is not possible to plan future capacity with accuracy. It can even get worse if the system is of limited size because the time spent on analysing and planning can quickly cover the savings.
The only way to have good predictions is to already have the systems running in production [for months], with a [mostly] static user base, running applications that don't evolve, and never add any additional service. Given these circumstances, we could have good metrics on current usage and make good prediction on the future... except that to be at this point the hardware has had to be bought already.
