Timings. The earliest the FA-50s could realistically enter service would be 2024. Remember that they have to be built, trained for, be introduced to service etc before the MiG-29s would be able to be released. I can't see the Poles giving up a capability , even the MiGs, without a replacement first secured and in/entering service. If anything, a batch of additional F-16s would be more likely to trigger a Polish MiG transfer than the FA-50s.
I didn't realize the timeline played out that far. Thanks for the info.