> Despite an excellent memory, I forget the names of most of my colleagues after a month's break. If I leave the company, they are never mentioned again. Apart from the usual exceptions, "standard" working relationships, especially in large companies, are tenuous, easily broken, and easily forgotten.

I think this is just normal human self-regulation. Extreme example, but people are mostly OK and functional a month after their mother passes away. If the best and most critical person in my team would leave today then I would focus on filling the gap, etc. and I would not think of them very much after a few days, but this doesn't mean that all that they're a fully fungible cog.