Except then there is no equipment to operate the Cap back westbound while it's equipment takes a tour of the south.
In the example you're thinking of, the Superliner Cardinal originated in CHI since there wasn't a Cardinal set on hand and CHI has a bunch of Superliner equipment on hand.
It is an unlikely scenario. However, if something were to occur where 20 could reach DC but EXTREMELY delayed than it could occur (for example a 24 hour late train). The consist of the arriving 30 usually sits in DC for more than 24 hours, so the Crescent's consist could arrive in time for 29.