I think the designers were more concerned about failures of components which would affect most of the DC network. E.g. Large short circuits on relatively unprotected sections of the bus system, which would affect a majority of DC-related systems if the Bus Ties were closed. Don't forget that smaller DC voltages are often used to control AC powered systems on most aircraft types, so the consequences are great.
I understand your point about losing large sections of the electrical system, but it's better than losing
all your electrical system
When bus isolation is initiated and, afterwards, a failure occurs on one side of the aircraft, the aircraft should remain in a controllable state, at least long enough for you to land.
Rgds
NSEU