The thought is that the chance of failure before 70 years was meant to be infinitesimally small, so the risk is close equal. 10 years from now a parallel replacement would have been contemplated, at which point the original could have been taken out of service and relined at a lower capacity.
The cost of two 5 foot pipes is likely way more than one large pipe.
Anyways, found the data set, which the city has in a nice visualizer.
Here are the concrete pipes installed between 1969 and 1981.
View attachment 573266
Water main assets owned and operated by the City of Calgary for the distribution of drinking water.
data.calgary.ca