The decline events are all restricted to happen between 1600 and 1700 so if you get lucky you can have no decline at all, or at least get away with just level 1 decline and still keep the full Janissary bonuses. I feel like these events could do with a bit of reworking, they're not very compelling right now and it's quite likely that they will never happen in the first place. At the very least the a lucky OE should have much better chance of the initial event firing, or even better, they should just start with it since the Janissaries were well established already by 1444.