You stated ' For example, different wired connections at the last mile are still commonly run through the same conduit. When that's the case and an event physically damages the conduit, it typically takes out both connections, leaving the business with an unacceptable amount of downtime until repairs are made.'
This is very important concept and last mile , aggregation or even core of the networks might have shared infrastucture. Generally because the same transport equipment , same fiber conduit etc. If they share the infrastructure , we call this as fate sharing but commonly this concept is known as Shared Risk Link Group or SRLG.
In the core of the networks, if there are more than two paths, IP and MPLS traffic engineering is used to diverge the path to non shared risk link group paths.
3G, Satellite, 4G , LTE all can be used as backup and even as load balanced.
If latency is high over 3G, 4G link compare to link which is considered as primary, applications which require high bandwith ( such as FTP ) can be sent through 3G,4G and applications which require low latency, low packet loss ratio ( such as voice and video ) can be send through primary link ( Ideally MPLS, DWDM,Dark Fiber , SDH ).