In practice, fibres don't just break randomly, they break because someone digs them up or the duct they're in collapses or similar. If any of those happens, the water line is going to be broken and need digging up in exactly the same way as if there was no fibre there. And in fact the water line might protect the fibre under some circumstances.

Fair point.

And I guess depending how it’s installed it may not be that hard to pull new fiber through without actually digging anything up, just from access at two ends.

I assume that's the point of the system - there are some long stretches of pipe at any place that pipe does anything there'll be some form of access point. So you get to the access point push fibre through to the next point (which could be a half a mile away in rural areas) and then do whatever termination you need. Repeat until broadband.