Urban air mobility (UAM) can bypass congested ground corridors and provide substantial travel-time savings even before its supporting vertiport network is sufficiently dense to minimize ground access and egress. We study this mismatch between service attractiveness and infrastructure maturity in a setting where ride-sourcing provides both the direct ground alternative and UAM feeder service. Passengers differ in direct ride-sourcing travel time, so UAM selectively attracts travelers facing relatively time-consuming ground trips. We develop a two-stage model in which vertiport density is chosen before fares and passenger mode choice and compare decentralized air-ground provision with platform integration and a no-UAM benchmark. When pickup and repositioning are sufficiently efficient, UAM reduces ride-sourcing vehicle-hours traveled even with sparse vertiport coverage. Its effect on vehicle-kilometers traveled (VKT), however, changes sign at a unique vertiport-density threshold: below the threshold, long feeder movements increase VKT; above it, UAM reduces VKT. At a fixed network, integration lowers the door-to-door UAM fare and magnifies the prevailing VKT effect by shifting more passengers toward UAM. When vertiport investment is endogenous, integration also raises the private return to coverage and induces a denser network. For intermediate deployment costs, only the integrated network crosses the VKT threshold, reversing the sign of UAM's mileage effect relative to decentralized operation. A New York City application exhibits the same reversal: integration increases VKT on a fixed two-vertiport network but makes a third vertiport privately worthwhile, yielding lower VKT once infrastructure responds. Deployment support and a UAM fare instrument address the remaining coverage and passenger-allocation gaps. These results show that the transportation effect of platform integration cannot be assessed independently of the infrastructure investment incentives it creates.