At the time, DCA-LGA on AA (IIRC) still ran close to hourly. I recall it being a big deal when they cut one midday frequency.Amtrak already carries 75% of NY-WAS air+rail traffic, even with the current "slow" schedules. You'll never eliminate all NY-WAS air traffic, because some people are flying to make connecting flights, or maybe their final destination is near the airport. Where they need to speed up the service is NY - BOS, and the first thing they need to do is get consistent 70 mph service between New Haven and New Rochelle, but that will require the cooperation of Metro North and the State of Connecticut. No maglevs needed.
[I'm also not sure if that 75% included onward connections on either side, but connecting traffic only needs 3-5x/day, not hourly service. It's where the market is /now/, but it wasn't there /then/. Also, killing the shuttle service would have given Amtrak more pricing pressure on the Acela side.]