Socialist economy with a growing financial hole, a massive demographic issue (by the time Poland shrinks to 37mil Germany is estimated to lose over six milion people and have a massive economic problem becuase so much of your population will be at retirement age it will threaten your economy) which brings us to illegal immigrants.
While Polish internal policies hamper family due to economic issues and as such are temporary German ones are social and are far longer lasting which means that at some point in the next 2 or 3 decades Poland will have a population surplus while Germany will be an aging nation needing people to work for the retired so you might see repeat of U.K Polish migration on a far bigger scale, this time to Eastern Germany, by the time its done German Neo-Nazi groups might find themselves ... outnumbered:)
Did i mention how heavily socialist you are while we are not?:) Then there's Turks who's family model currently is 2+5.
If you want proof check the registries of the German border towns, some of them have as much as 50% Polish populations and growing.
So i should rephrase, imminent VISIBLE decline because for now its still relatively low key.