Maybe I'm lucky, but whenever I've been in a position to have a potential "trip in vain" circumstance, UA customer service people have made diligent attempts to find alternatives. Sometimes they have failed due to lack of viable alternatives, other times they have come through.

A few years back I had an issue with a routing LAX-ORD-FRA. Needed to be in FRA for a morning meeting, and had a business class award ticket. A last minute mechanical made it impossible to make my ORD-FRA connection. The LAX agent put me on the LH non-stop which had one open seat in business (or course I had to hang around the airport for six hours). In order to capture the seat the agent had to book me full fare revenue. At least on paper, that cost UA just over $3,000. Got a lot of nice bonus miles from that mechanical!