The nylon lines are only a safety system - there's typically a mainline and then only branches off of it. The only time multiple lines are used are at the entrance (the mainline is usually placed 50-100m back to prevent untrained divers from finding it accidentally) or where there are jumps, and in both cases the temporary connections are retrieved when the team of divers exits.
In some higher traffic caves these lines are replaced regularly, however with most caves it's an every couple of year type situation and done by volunteer teams.
As a cave is explored & mapped, line is tied on to the end from the explorer's reels - adding multiple knots and failure points. Once there's sufficient traffic or need usually caretakers will go through and attach more resilient line (usually a spectra or dyneema line) and take out the original line, only leaving the exploration markers (name, date, expedition, etc). These teams also find more permanent attachment points or add them (using silt anchors and sometimes bolts) so that the lines aren't secured to cave features wherever possible.
Adding & removing lines on each dive would actually cause more damage to cave features than keeping a permanent or semi-permanent line in place, and there'd be a much higher risk of line movement and entrapment as the connections would be fewer and further between.
In some higher traffic caves these lines are replaced regularly, however with most caves it's an every couple of year type situation and done by volunteer teams.
As a cave is explored & mapped, line is tied on to the end from the explorer's reels - adding multiple knots and failure points. Once there's sufficient traffic or need usually caretakers will go through and attach more resilient line (usually a spectra or dyneema line) and take out the original line, only leaving the exploration markers (name, date, expedition, etc). These teams also find more permanent attachment points or add them (using silt anchors and sometimes bolts) so that the lines aren't secured to cave features wherever possible.
Adding & removing lines on each dive would actually cause more damage to cave features than keeping a permanent or semi-permanent line in place, and there'd be a much higher risk of line movement and entrapment as the connections would be fewer and further between.