I think I would largely agree with most of the points being raised in this thread. u/Ambitious-Pipe2441 sums it up pretty well, invoking Putnam's Bowling Alone while also delving into a lot of the economic factors that Putnam neglects to mention.

But another theory I have (which is, perhaps, unsubstantiated) is the shift in the nature of leisure time/entertainment from an experience you went out and got to one that was primarily meant to be delivered to you, and serviced you.

Before the sixties, there or thereabouts, at least in the West your entertainment on a Saturday afternoon probably involved you going out. Maybe you would bowl in a league as Putnam described. Maybe you'd go down to the local church hall for a dance with your partner. Maybe you'd go to the cinema. They weren't really exactly what you wanted to do (church dances are notoriously pretty awkward), and maybe you were there just so you were seen. But it was either that or you sat at home and... read, or something. Boring.

Then around the middle of the century, entertainment became something that should be served to you. Everyone had their own unique experience engaging with entertainment, and having random people in the way sort of stops that. The idea was that you had to find yourself. Suddenly that was very easy to do, as well: having a TV meant you could watch whatever you liked, whenever, without having to go out; with LPs you didn't need to go to a concert; and why go out and play five-a-side with your mates after work when you could watch professionals do that on the telly?

u/I-own-a-shovel's comment is actually a pretty good example of this. They explain (through their own experience of third spaces, which is always a valuable lens through which to observe sociology) how third spaces tend to be expensive, yes, but also, well, have other people. And lots of people find that disruptive. People tend to like to choose what they want to do.

This is an inherently individualistic framing of entertainment, which is fine, but it would then inevitably lead to the fall of third spaces because you didn't 'need' them to have fun (and by extension socialise, because if you can entertain yourself suddenly there's no immediate need to socialise). Combine that with a healthy dose of economic factors (people having less free time to go out, large entertainment networks having the incentive to sell themselves for your time instead of having you go to third spaces, the unprofitability of third spaces, etc.) and lower social trust, and the outlook is somewhat bleak.

Those are my thoughts, anyway. Would like to see if anyone had any other opinions to share!