Originally the manufacturers had to set an anticipated lifespan for the bags. They contain some powerfull chemicals and how much longer than 20 years can one originally plan.
As the original time frames approached MB found that they had enough data to decide they could go another twenty years. So as of some time a few years ago those dates were put off.
There are prescribed checks to be done in major servicing but the bags don't need wholesale replacement as of yet.
Bosch Master, ASE Master, L1
33 years MB technician