You are spot on if you are talking about buying ads on social networks, but that is not the real strength of social.
There is a difference between building an audience on social networks and most of the other platforms you mention. With the exception of email marketing, the other platforms are largely a one-and-done thing. You pay to run an ad, and it runs once, or however many times. You expect the payoff it be near immediate, even if it is just to build brand awareness. The ad cannot reach people after it is run. Email is sort of in between those platforms and social, since you need to build a mailing list, unless you are buying one.
Building an audience on social media is more of a long term game. The spend, be it money or time, is for the right hook that may be months or years down the line. The focus of most interactions is to build a following (width or depth) rather than a transactional “buy now.”
Since most social media awareness is often bought with our most precious resource, time, I think it is definitely a useful question whether that investment could be better made elsewhere. That answer will be different for everyone.
There are still benefits, even if a platform is going to fade away, since you can use those platforms to drive people to your website, blog, or other social channels. However, I believe that due to the nature of social media, it is a very relevant question whether a platform will be around long term.