Those “Only Alive” bounties would prevent people from trying to kill the Yonkos’ if they somehow got the chance because if they would kill them they would not receive the huge bounty on their heads which the WG would on purpose decide would be so non-proportionally huge so they’ll both be seen as massive threats to the public AND that people would prevent themselves from attempting to kill them so they can receive the money from the bounty.
At the beginning I thought that perhaps they would not even have bounties but the WG needs them for propaganda purposes and the purposes I mentioned to indeed the Yonko would have massive bounties just because they got to this amount of power, the bounties help them preserve that power and keep the balance.