And 'impossible' is used simply because of what has been going in the area for some 80 years. I doubt whether anyone would be willing to go into the pit and repair fallen brickwork, and certainly not for a charge that most people could afford. If the OP is willing to undertake this work, I will pass his details on to the people with these pits in this road.
If it did begin to disintegrate, I could hardly expect someone to come round and repair it in view of what's been continually poured into it for some 80 years. Surely physical contact would be a health hazard?
Of course somebody will come round to repair/enter it. It won't be easy or cheap and it's not something I'd like to do, but sewerage work is a trade like any other and somebody will do it. On TV a while back (can't remember what programme) they had a diver who specialised in diving to the bottom of the big tanks at sewage treatment works to do repairs - if theres a job to do somebody will do it.