A long term stagnating economy and especially a shrinking one would be considered an economic disaster anyway. The key is to avoid social problems due to shrinking the economy. There are plenty of ways of doing that, but propably the most important one is to limit working hours. That means earlier retirment, less hours worked per week and more vacation. All of that are things mandated by countries before. With a globally stagnant population that might very well happen. In fact you see falling working hours in Europe over decades.