why do we need an incentive to break PGP?

Let me explain, the basics:

Any system that has a case where key collisions can occur less than random is the only incentive you need...large use of bitcoin ensures that case all by itself with the rule that you use a new key for every transaction.

LEOs use this track down illicit money.

crypto in practical terms is not secure in that the key collision when it occurs and it will unmasks you not directly but indirectly and the cost to do the work of that lowers as it ages and does not go up computation wise.

It's somewhat perverse where when the world uses more bitcoin the more key collisions occur.