The former could still be cheaper than stop using libgit altogether.

No one is asking them to stop using libgit2 though. They’re going to continue using it. If they find a serious bug, they’ll fix it and continue using it.

The cost of all the additional hardware is just not worth it. If it was a choice between higher hardware costs, higher request latency, greater operational complexity of a new technology and rewriting libgit2 in a different language without all those tradeoffs, GitHub definitely chooses the latter.

But it’s never going to reach that point because they’ll continue using libgit2 compiled by clang forever.