In another forum, two United pilots discussed this flight and the diversion. If it was deemed that the CA could not continue the flight as PIC, the two FOs could not have continued the flight from MIA to ORD. The flight would remain on the ground in Miami until a replacement CA was flown in. The requirement that a CA be PIC of the flight is part of the union contract. (If the CA had become ill, the FOs could continue to fly the aircraft either to its destination or to a diversion airport. However, once on the ground at a diversion airport, a new CA would need to be placed in the cockpit to continue the flight.)
The FOs ability to affect the decision on whether the CA could continue piloting the aircraft was limited to their assessment on whether the safety of the flight might be compromised by having him remain as CA.
Mention was also made that the CA is more likely to be on medical leave rather than being suspended, which lends credence to the rumour that the CA may be seeking a disability-based pension.