Why do you consider this to be a mistake? I've thought about this in the past, but can't really articulate my motivations, and in the end I haven't followed through. Some of my previous colleagues have fit into the gap between co-worker and friend. I would never spend time with them outside of work and since we no longer work together I'm not sure what I'm really missing, so I'm curious to hear your thoughts.
Those connections become invaluable as you reach the point where you are responsible for putting a new team together / growing one.
Opportunity loss. I came across the phrase "increase your surface area of luck" somewhere and it is relevant here.
I am guilty of the same.
Happy birthday? That seems unnecessary really you don't need to send cards to all your connections. Just reach out if you want to reconnect don't overthink it - I feel it's pretty normal to drift in and out of connection/friendship even.
Yup! Not everyone will jump at it, but most will. I let some connections lapse for 10 years and then reached out. Ended up with some great conversations, good advice, and two job leads (from the hiring managers themselves.)