Why hasn't BGP completely replaced OSPF and IsIs? What do the other two protocols handle that BGP does not already implement?
In what circumstances would it ever be practical for BGP to replace OSPF or ISIS?
BGP is an Exterior Gateway Protocol (EGP), it does not understand items like the bandwidth of links. Compare this to any Interior Gateway Protocol (IGP), including the two you mentioned, that make their routing decisions partly on available link speed.
BGP is more complex to configure properly than any IGP, add on the lack of support by lower end routers (not just bottom end routers) and the lack of automatic neighbour discovery and it becomes plain why BGP isn't about to take over any time soon, or ever in fact.