This is the real concern—there simply isn't enough ridership potential. (and I'm a huge proponent of MOAR TRAINS)

Alon Levy covers the topic a lot on their blog. A critical point is that "any air-rail link must go to the areas that people are likely to want to connect to." The Pittsburgh metro is not monocentric, so there's no easy way to bring everyone to where they need to go next.

People assume that airport transit would get used a lot, but it's orders of magnitude lower than everyday transit riders. The most frequent users would be airport employees, and even at bigger airports there usually aren't enough employee commuters to make it worth it.

Marks against a PIT train:

• small airport
• far airport
• nothing in between the airport and the central city
• small metro region
• weakly centered urban development plan

Instead, we should focus on "duh" improvements to bus transit to the airport:

• dedicated airport traveler-friendly busses (with luggage racks like Boston's Silver Line)
• run more frequently
• don't make non-airport stops
• take advantage of existing bus rapid transit features (like the East Busway)
• run a few variants (like, 3—maybe northside, east end, and southside)

In parallel, we need to build up transit capacity and reliability in the core so that it's the default way to get around.