With only one station, there will be a lot of trains stuck in the tunnel when they need to hold service. Evacuation is going to be annoying too if someone jumps at Kennedy.
Another problem with this extension is the conversion of Line 2 to ATO. It only makes sense to go with new technology meaning TRs will have to move to Line 2 with the opening. If no funding is available to replace the T1's by 2023, this would be a good excuse to delay the extension or install the current block system and ripe it out in a few years. If they do replace the T1's, then they'll replace them at a premature age with another 5-10 years of life. It's going to be gravy either way.