How often do sewer lines collapse?
Collapses are just one problem with clay sewers. Probably more common are roots clogging the pipe.  If they do collapse, it can occur under your foundation slab, or out in your yard.

Some 60 year old clay sewers might still be in great condition while others not so great. If you want to know the condition of yours then get a video inspection.

Do you have a basement and is it finished?  Obviously a finished basement is a lot more costly to repair than an unfinished one.  If a collapse does occur, the backup will occur pretty slowly (versus from the city where you might get a few feet of water in your basement quickly). Water alarms can be obtained relatively cheaply and would alert you to the issue early on.

Ultimately you need to weigh the cost of the insurance against the cost of repair.  Remember that insurance companies don't make their billions by paying out claims to people.