The main reason you do not want to replace a FL with a circuit breaker or fuse is very simple. Safety. FL are designed to prevent fires caused by battery short.. .Very High current potential. Rhe deal is wgen there is a high current short the likelyhood of sustained ionization (arching) is much higher than for the typical low curent circuit.
Circuit breakers and fuses are not physicaly larger enough to prevent internal arching. The arch in this situation is capable of carrying high current. Hence the circuit is not fully opend fast enough. ... The result is a potential fire and having your battery cables welded to the frame.
FL's are longer and extinguish the arching much faster reducing the potential of fire. Also the arching that does exist in the length of the FL is longer and hence of higher resistence which reduces the current dramically even before the circuit is opened fully.
This topic seems to come up every couple of months and the physics still has not changed.
Steve