The game was actually close to ship without multiplayer, it was an afterthought. Some devs have talked about this (iirc, Marty O’Donnell was one of them).
Goldeneye is one of the best case studies for why companies should give their engineers room to experiment. If I remember correctly, the multiplayer mode was originally a side project of one of the engineers and was added near the end of development.
This isn't to say the campaign mode isn't good, but the ability for boys to play a shooter game against each other on the same console is what made that game.