The weak link here is not the lead itself but the rubber insulation, and also the long history of wear and tear and damage on any installation that old (even if the cable itself is perfect). The airtight lead sheath actually helps to preserve the rubber but it will still deteriorate at the exposed ends just as fast. Rubber insulation ages in unpredictable ways due to subtle differences in the formulation and processing. Some might be OK for another 70 years, some might already be internally disintegrated to dust. I have some unused lead-sheathed rubber-insulated cable from the 1930s that is as good as new; the insulation is tough and supple and off the scale for IR, but there's no telling how long it will last.
Another problem with lead in domestic installations is that the sheath was normally used as the CPC (although there were a few types with a copper CPC). Continuity was maintained at junction boxes and fittings by screw clamps, but the lead tends to flow over time and the clamps become slack, resulting in uncertain continuity.
Lead-sheathed paper-insulated cables are still in widespread use, e.g. DNO distribution and service cables, which can be sound at 100 years old or more and might outlast plastic-insulated cables we're installing now. But those are sealed and impregnated with compound that acts as a preservative as well as an insulator.
