I'm sure a lot of people don't have the kids with the purpose of not being alone in old age. But I also get that there's some kind of "soft expectation", for lack of a better term, that your "family" will kind of stick around, especially when you're alone because raising said family more or less led to a falling out with old friends. There's also the factor that when people get old, their own friends may have gotten old and died.
Where I live I don't see many "intergenerational friendships". Granted, I'm not a very social person so I don't really seek the society of others, but from what I see around me people tend to hang out with people of roughly the same age. There also seem to be a lot of loose friendships, in that people would get together more or less often, have a good time &c, but those are not relationships one would be able to depend on in case of a problem.
I think your point about fenced-off suburbs is quite important though. I figure that once people get older, it's much easier for them to try to hang out with other people if they all live more or less close-by. Such as joining some kind of club. Even if it won't become some very close friendship, at least they're less likely to feel as lonely as people in the suburbs.
I think that both of these are massively consequence of our own culture, not something that must be.
It is combination of long hours at work, the super high expectations on child raising that for many people excludes socialization with kids present and animosity of childless toward kids present near "adult places". And vice versa.
It is also a result of socialization being seen as slacking and not doing useful things. It is also result of sort stigmatising the behavior that leads to keeping long term friendships (at least I had to figure out this is issue for me).
On the balance I'm sure I'm more social now (corona notwithstanding) than I was 5 years ago.
Even more so when people have to juggle kids and a career.
I have a wife and 2 (small) children. There is nobody in the world I love more than my family - it used to be only my wife previously and now grew from 1 person to 3. This has such a profound effect on my happiness now (in my 30s) that even if I die suddenly at age 55 (long before becoming a lonely old man) it would still have been worth it.
Yes you should also have friends & if they are of various ages that's even better. You dont need tech to do that either, although it is probably difficult if you live somewhere far from other people (and some developments like fenced off suburbs with no city centers make that harder still). But that is independent of the argument if you should or should not also have kids.
You can have kids and friends!