Numbers don't make sense
"It says it has built up an annual run rate of more than $200m in subscriptions.
GitHub is said to carry at least a $5bn price tag after it was last valued at $2bn during its latest funding round in 2015."
Valued at $2bn, asking price is $5bn and annual income is around £200m. So not only is ROI, even at 0% interest going to be around 25 years, that doesn't even take into account the numbers of people who will leave if MS buys it. New customers moving to a possible MS GitHub are likely to be much lower numbers than those leaving IMHO