The reality in most cases is that if a reg fails, it will fail open and freeflow - stillproviding the diver with air for ascent. Failures where the reg fails closed are very rare and if they occur usually involve the second stage, where the octo can be used to avoid a life threatening situation as you can still get air.
I have encountered two second stage "fail closed: situations in 25 years of diving. One was due to ice forming in the second stage during an ice dive and occurred in a full face mask. The second involved an R109 I had serviced where the lever slipped off the poppet - a very rare event in what is a bullet proof reg. But it was probably caused by a 30 year old lever that had had seen too much use and had prongs that were too easily bent out of alignment.
I have never encountered a first stage that has failed closed. The most common failure mode is a slow leak, and if you ignore them long enough, they can get big.
So in 99.9% of the cases where a reg failure occurs, it is inconvenient rather than dangerous if the diver is properly trained and diving with a buddy or is redundantly configured. Even in cave diving, if a diver has a failed reg, they train to deal with the failure to prevent/limit gas loss and switch to a backup, so again it is an inconvenience, not truly a life threatening situation.
That does not mean you can play fast and loose with regulator maintenence, but it does take it out of the "oh my god yer gonna die!" category.
Personally I got into reg repair as I was tired of the LDS screwing it up where I lived (poor adjustment one year, damaged piston and resulting HP seat leak the next and a tear in the diaphragm in the third annual service - they NEVER got it done right). And as I was advancing into techncial diving, I wanted more reliability (and could forsee the huge costs of relying on a shop to do them all - especially with the outrageous rates charged for O2 cleaning for O2 regs.)
I have encountered two second stage "fail closed: situations in 25 years of diving. One was due to ice forming in the second stage during an ice dive and occurred in a full face mask. The second involved an R109 I had serviced where the lever slipped off the poppet - a very rare event in what is a bullet proof reg. But it was probably caused by a 30 year old lever that had had seen too much use and had prongs that were too easily bent out of alignment.
I have never encountered a first stage that has failed closed. The most common failure mode is a slow leak, and if you ignore them long enough, they can get big.
So in 99.9% of the cases where a reg failure occurs, it is inconvenient rather than dangerous if the diver is properly trained and diving with a buddy or is redundantly configured. Even in cave diving, if a diver has a failed reg, they train to deal with the failure to prevent/limit gas loss and switch to a backup, so again it is an inconvenience, not truly a life threatening situation.
That does not mean you can play fast and loose with regulator maintenence, but it does take it out of the "oh my god yer gonna die!" category.
Personally I got into reg repair as I was tired of the LDS screwing it up where I lived (poor adjustment one year, damaged piston and resulting HP seat leak the next and a tear in the diaphragm in the third annual service - they NEVER got it done right). And as I was advancing into techncial diving, I wanted more reliability (and could forsee the huge costs of relying on a shop to do them all - especially with the outrageous rates charged for O2 cleaning for O2 regs.)
1+ what Thal and awap said.
