I propose that the life expectancy of a GP is directly related to how “long” power is applied to the plug… “long” in the sense of each cycle that the automated relay controls based upon temperature of the engine and of course “long” in the sense of total “powered on” time the plug accumulates over long use.

Thus IF/WHEN I am faced with a major expense with my GP system [e.g. a relay replacement], I will likely opt to “retro-grade” my GP system with a manual relay that allows me to control when and how long I use my GP(s). Such a manual relay “retro-grade” would also cost far less than the “Up-Grade” kit from Bosch.

P.S. - I just drove a 1.5L turbo diesel Renault "Megrane" for 3-weeks in Italy and not only do these new technology diesels have a very short GP cycle, but it is totally controlled by the engine control system and their electronic ignition key card. God bless these simple vintage MBz(s)!
Regards,