Why is it that folks who point out that Facebook, Twitter, and Square all had to move to a "fast" language never seem to be able to point to any company (in the current generation) that achieved massive success by starting in a fast language? Having to transition to another language once you achieve success is a pretty solvable problem in comparison to actually creating a product successful enough that platform matters.

For that matter, neither Square nor Twitter have exactly "migrated" (I don't know about Facebook). They optimized hotspots -- a long and storied tradition. Even C programmers drop down into assembly.

Use different pieces for what they're best at.