The F-16 is a very good fit for Polish needs with a vast array of weapon clearances with a clear development path and predictable maintenance & sustainment costs. That they are relatively cheap will not upset the bean-counters and as mentioned above you can expect quite a lot of political good will from the USG.
Whilst Typhoon and Gripen are highly capable aircraft don't underestimate the advantages of buying into such a large F-16 community and the speed at which the USAF system can train a few squadrons worth of pilots and maintainers. Of course, when your requirements can be satisfied with secondhand aircraft there are very few aircraft types out there that you can rapidly buy around 100 aircraft - the Typhoon and Gripen fleets just do not have that number of spare aircraft kicking around.