I honestly don't see any downsides to this, for either Google or Apple.
In years past, a person who was a heavy user of Google services either had to choose between keeping those on an Android phone, or give them up if they really wanted an iPhone.
Now, they don't have to choose. They can get full use of Google services on either platform. Win for Google, and win for Apple for not making users choose between Apple's hardware and Google's services.
It's not like Apple is competing for the same services as Google. While there are a few Apple equivalent versions of some Google services, I don't believe Apple loses money due to people using Chrome, Gmail, Google Now, and Maps on an iPhone, do they? If people are buying iPhones and using Chrome over Safari, Drive and Gmail over iCloud, etc does Apple even take a revenue hit over that?
Yes, I know that a lot of people here don't use Google's services, and you're not being forced to. However there's more than a few that do, and are now able to give Apple money for their devices without having to give those services up. What's the downside?