I'm the OR from that post who has been in our small Manhattan apartment with two kids (teen/tween) this whole time. My two closest friends, both wealthier, fled to their country homes in March and haven't been back (their privates are virtual until Oct at least). It's not that I feel resentful so much as I feel like they simply don't live here anymore. For the first couple of months we would FaceTime or text regularly and I looked forward to their return. Now I feel like we just have nothing to talk about - it's like a breakup, I've moved on with my life. Anyone else?

We've been here the whole time and haven't seen any of our friends. I'll be honest--we haven't felt the need to. We are enjoying the family time. It's not about money or employment situation or even location.
Every year I've lived in NYC my friends have moved away. I consider it part of living here, and one of the downsides. It has been difficult for me to keep up, but if I'm ever in their neighborhood, they're happy to see me even if we haven't spoken in years.
I ran into people this weekend I haven't seen for 6 months, and we just stepped back into our normal routine. It's fine.
Because things HAVE changed and you are completely valid in your feelings.
Maybe these friendships are more situationally dependent, like your kids are friends or you worked together. These kinds of friendhships tend to fade when the common denominator is removed.
You say you're not "resentful," then why comment on how much money they have? Would it be different if they were not "wealthy" but left the city?