Tim, my experiences have been different, albeit mixed - but the key is to make the relationships about something other than work if you want to keep them. Super easy to keep things going while you're working together as there is a shared experience to always kick-off the conversation. Once that goes away, the relationships wither and die.
Meet some folks at work that you really enjoy? Those people you would choose as friends (and vice versa) anyway? Then don't have conversations revolve around work. I'm always quick to say "you know, we can talk about that report that's due tomorrow when we're on the clock - this is our time - so tell my about your 8-year old daughter". I still keep in touch with people I worked with 20 years ago. Flew into a town I worked in over a year ago and had my weekend packed - and bonus points for not having to pay for a single drink or meal!
Point of my post Tim, is not to gloat or say that anyone is wrong - b/c 100% you/we can be replaced at the drop of a hat. Those relationships that happened automatically and organically definitely will drop off for the very reasons you listed. In my experience, it's exactly the same with old school friends. If all you have is the shared experience, once you lose that, the relationship has no roots. Life does get in the way and "let's meet up for drinks after work" becomes "I've got some time 3 weeks from Tuesday between 5 and 6:15".
Circling back on that wonderful weekend I mentioned with the full schedule? To be perfectly honest, when I quit I was disappointed that virtually no one reached out. I'm sure I was replaced in a few weeks with someone even better. What changed was simply taking control of my half of the relationships. What works for me is to reach out a couple of times and only move on if it becomes clear the other person already has - no regrets - and take the good times while flushing the bad.
"Don't collect business connections like empty beer bottles" is 100% correct - my goal has been to make sure and leave something in the bottle if I can.
