The main issue seems to be that Uber isn't replacing taxis but augmenting them: before there were about 2000 taxis in the core of Manhattan during peak hours, and there are now 2000 taxis and about 1900 Ubers. The trips per taxi seem to be slightly reduced, but overall there's still an increase in the total number of taxi-like vehicles circulating, and the total number of trips made by automobile vs. other modalities (subway, walking, etc.). Thereby making it easier to get a taxi, but making traffic congestion worse.
The main issue seems to be that Uber isn't replacing taxis but augmenting them: before there were about 2000 taxis in the core of Manhattan during peak hours, and there are now 2000 taxis and about 1900 Ubers. The trips per taxi seem to be slightly reduced, but overall there's still an increase in the total number of taxi-like vehicles circulating, and the total number of trips made by automobile vs. other modalities (subway, walking, etc.). Thereby making it easier to get a taxi, but making traffic congestion worse.