You discard that as if nobody needed elasticity.
Have you ever dealt with a large B2C site? Traffic tends to be heavily seasonal there, and in many other genres, too.
"Capacity planning" is a nice word for what in reality commonly boils down a bit of educated guessing, followed by the decision to keep a $reasonable amount of over-capacity around. The exact value of $reasonable is hardly ever specified by much more than the equivalent of a dice-roll.
Moreover, the interesting question is not which provider can provision a pile of metal within 4 hours. The interesting question is which of them will take those machines back a day later, without charging for a full month. A cloud will do that. Will your managed ISP?