In general, my impression has been that you don't want to architect your solution at first for massive scaling, because:

* You probably aren't going to need it, so putting the effort into scaling means slowing down your delivery of the very features that would make customers want your solution.

* It typically slows down performance of individual features.

* It definitely significant increases the complexity of your solution (and probably the user-facing tooling as well).

* It is difficult to achieve until you have the live traffic to test your approach.