Cartridge Bottom Brackets are disposable units, Shimano make Octalinks in ES25 & 51 versions, both have plastic shells, both are perfectly usable, if Shimano thought that metal was needed for this part, they would have spec'd it, they have been in this business longer than just about everyone else, and know what they are doing.

Not sure what you intend to replace it with, as nowadays, only Shimano make Octalink BB's.