I see too many startups burning time/money on scalability they dont need. If you have 1000 customers, and your rack can handle 20000, dont talk to me about future-proofing against the day you are building your own datacenter. Keep your code working for today's customers. Worry about scale when it happens ... IF it happens.
Imho, limit your perspective and planning to double your current customer base (good day senario) and half (bad day senario). Any day beyond those bounds will require a rethink. A rethink THEN, not today.