Many other countries are also non-extradition to the US, for various reasons including reciprocity and how the US treats its criminals. It’s definitely not unique to Cuba.

I wonder how long before Mexico and Canada (and maybe others?) repudiate any extradition treaties they might have with the US.

Presumably hijackers would just go to the next nearest country with no extradition policy. Honestly though I’ve never heard of “preventing hijackers” proposed as a reason for wanting to normalize relations with communist Cuba. You realize there are no hijackers anymore right? Additionally, there are many US citizens and corporations with valid and verifiable claims to property confiscated by the communist government in Cuba. Normalizing relations would obviously require a resolution to this situation.