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.
Yeah I think there is a lot of truth here. You can't solve all the problems and in Heroku's case we focused on user experience (internally and externally). Great ideas like "git push heroku main" are game changers, but what happens once that git server is receiving 1000 pushes a minute? Totally different thought process.
Perhaps the thing I would add is that even with the tech debt and scaling problems we still had over a million applications deployed ready for that request to hit them.
* 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.