Pigeons do not die right after mating as that's just a misconception.
Pigeons even when mating will live a normal lifespan of 15 years if they do not get harmed in any other way or come down with any deadly diseases.
If you notice a pigeon dying after mating then the pigeon likely had some other illness or was just old and ready to die off.
Pigeons will continue to mate with other Pigeons for it's entire lifespan.