Don't be daft otherwise every other nation with abortion rights would have a falling population rate as well.
Something is causing the birth rate to drop. You can't blame abortion for that any more than you could blame the availability of birth control. If we take your argument at face value then Germany should ban the pill, condoms, sterilisation and all other birth control methods until the population starts to grow again.
You can't compel women to have children. Perhaps Germany should look for the reasons why fewer women want fewer babies.