I like your analogy of a garden/friendships and I agree with neglecting friendships means like a plant, they wither and die if not tended to.
My "etched out lives" comment about women in our 50s, is that by that age, most women have established friendships with other women and don't prioritize room for new friendships. That is a generalization of course, based on my own projection of being 52 with few friends and many acquaintances.