Or indeed Google develops a competitor themselves if Uber and Lyft gain 9-digit valuations. It could do that in less than a year and for a few hundred millions at most. Taking a page out of Apple's book of vertical integration and squashing popular services with in-house offerings.
That assumes that the feature in Maps not only exists but is widely used by consumers in preference to individual service apps; my impression is that that is not really the case.
It's a two sided market that takes a significant investment to create. Yes, Google can add G-Car to maps, but where are the drivers coming from? They're not going to switch unless they can expect more income than from Lyft/Uber. Google can't provide that without investing a lot of money in incentives.
The best analog to Lyft/Uber is Mastercard/Visa. It's theoretically easy to enter the market, but in practice it's going to be very difficult to develop your own two sided marketplace going against the incumbent competitors.