When my sister's MIL died, she and her FIl had done anything and everything as a couple. He didn't know how to do anything alone and wanted his son and my sister to fill the gap - which my sister wasn't happy about. When our father died, I think Mum found it a bit easier because she already had her own social life and social circle- my parents had always done their own things, as well as things together, and I think that's a good thing.
It's definitely something I expect in my own relationships. I don't have many local friends, but in my 50s, I still catch up with people from school, uni, houseshares, past jobs, and when I've been in relationships, I expect my partner to have their own friends, because it's what's normal to me.
I was at a party before Christmas, talking to someone about how we knew the hosts - he lived down the road from them. I met them in our first post-uni job - they houseshared with my then boyfriend. We're not really close, which is partly a result of geography and moving for jobs, but there are various levels of friendship, and I think men aren't as likely to maintain more distant ones. The man I was talking to was amazed we'd known each other so long, whereas the week before, I'd been back in my childhood home catching up with women I'd known since about age 3 in one case. He wasn't in touch with anyone from school or uni. Whereas at my father's funeral, one of his schoolfriends did a reading - they'd met age 7. So it's normal to me that you maintain relationships.
I've made friends all through my life. Different friends fulfill different needs - sometimes they're there for shared humour, or because they were there when things were tough, or for shared history etc. No one person can be everything.