When a team is dependant on one super developer

Hello 👋 I am observing a team that has 5 developers. One of them created the whole front and back end architecture and all the APIs.

Only issue, none of the 4 developers understand the system well and are super slow at developing anything. They received examples, but they are continuously facing hurdles while doing the work. They are super frustrated and wanted to use an open source framework instead.

The business has been too invested into this one “super developer”, the business doesn’t believe that they can back track and use an open source framework.

Missed deadline, growing frustrations, the entire team motivation has hit the floor.

To all accomplished developers and experts out there, what is your take on a situation like this? Have you experience something similar? What was your experience?