I do not follow this logic. They have enormously succeeded from the moment of Github's inception to this point. Does it not make sense to invest in them if they believe they can utilize new capital to expand their platform?
In any case the money believes in them. And they are already vetted.
They've never been a capital-intensive business, so it doesn't make great sense to invest substantial sums. Its more likely to change the business to one focused on maximizing profit, rather than improve the product.
Try the analogy where a startup is like a band (I'm not the first to use this). After the band sells out, do you expect better music? As the cliche goes, their artistic integrity is compromised, the members start in-fighting, and the lead singer grows increasingly strung-out on fame and drugs.
If anything, innovation on their product is going to speed up with the funding, then slow down because they hire a ton of engineers who need to be ramped. Then they will plateau.
With funding comes expectations. Github has never had to answer to anyone but their community and customers (and their board, but the board had no real monetary stake in the company outside of stock). Now A16Z has some control over their operations. Not sure what their terms were in the deal, but investment and board members change how a company operates.