Somewhere up the food chain there was a pinch point with no alternative routing available; in my book that really isn't good enough. Of course compensation for loss of service isn't provided either.
Eventually your single Zen connection is going to have to connect into a single device providing connectivity. If that fails then you're going to be without service until it gets replaced. This is the same with any other ISP, and the only way around it is to provide a second connection into your premises that connects to a different device.
Providing completely diverse paths into an exchange means you are doubling your backhaul costs. This may not be cost effective, especially with low numbers of subscribers on an exchange. Additionally a lot of exchanges - especially the smaller ones - only have a single duct going into them. This means that is pretty much impossible to provide any serious kind of redundancy as Jim with his JCB can cut through all the fibres going through that single duct in one fell swoop.
At the end of the day consumer grade DSL services are cheap. They will suffer outages from time to time. If you require guaranteed service (although it appears you can get by when suffering an outage) then you need a second circuit, ideally going to a completely separate exchange.