I think that most people think that starting a walled garden is easier when it's connected to an internet that everyone is always connected to.
The internet is different from the PSTN network that connected you to CompuServe because if Facebook controlled your last mile the way AOL did, you'd need to coordinate with Facebook when you wanted to upgrade to the latest DSL or cable or what have you. The idea here is that once we got DSL and other "run gigabit ethernet over a series of rusty coat hangers that we have twisted together" level technologies, (I think it's fucking amazing that DSL works at all) in one location, one technology is going to work way better than others, and those other technologies might work way better in other locations. In walla walla you might get twice the speed on compuserve, while in San Jose, AOL might give you 4x the speed, if AOL and CompuServe ran their own networks. (to torture the metaphor)
Of course, if you made the last mile technologies like DSL and Cable switch the endpoints of their connections as quickly as you can switch endpoints on a PSTN, sure, that would have solved the problem, too. But... really, that's kind of what the internet does.
I think this is why walled gardens that include the last mile keep failing; the last mile is largely hard based on physical geography, and the walled gardens are hard based on your preferences, which often don't match up with your geography.
I think that's why walled gardens are doing so much better now than they were in the '90s; these days, one set of companies focuses on the last mile, (and everyone has, comparatively speaking, astoundingly good last-mile access) and then a completely different set of companies can set up walled gardens to cater to various preferences, and the people that like that walled garden don't have to share a geography.