I agree, no horizontally-scalable system is infinitely scalable in any dimension. You will hit some limit - for example, you might be able to support arbitrary

I agree, no horizontally-scalable system is infinitely scalable in any dimension. You will hit some limit - for example, you might be able to support arbitrary scale in some dimension, but there is a bound on the sustainable rate of change, or the second-order rate of change, and you hit that limit. You seem to be suggesting though that hitting those limits came as a surprise, and/or there was no future architecture plan to address that when it happens, which I think is the real problem here. You would expect a team with the maturity of Github to understand what those limits are, and plan for them, based on predictable growth in demand.