How is people moving on after playing a game for many hours an issue? Most games have its fare share of players moving on regularly. Especially if you have played a game for hundreds, never mind thousands, of hours you do not really need any kind of justification if you want to stop.
Players come on go. Some leave indefinitely, others return after some time or to check something new, and then you also have altogether new players joining. In any game, not just Elite.
For Frontier, it's only an issue if more people are leaving than joining. In fact, one might argue it's a good thing for Frontier when veteran players who already own all the cosmetics move on, since those players are no longer making Frontier any money. New fresh players who are buying the game and purchasing Arx are the best thing from a financial perspective. Though I think Frontier would like to keep influencers around as that's free advertising.
The answer to "how is that an issue?" is very different on a personal level, however, when your community moves on, leaving you alone in the galaxy. This obviously only affects social players, but it's a real problem that can easily avalanche. For example, over on the Space Engineers (proving your "not just Elite" argument) server I used to play on daily, we had a close-knit community that really gelled together in that sandbox world to make the game feel alive. Even the "bad guys" (pirates and raiders) added to the feel of our little universe. Then one day the servers crashed (Keen's servers are utter rubbish) for a couple of weeks, and most of the crew moved on either to different servers or an entirely different game. After that, it was never the same. Something truly special was lost, and since then I haven't played anywhere near as much as I used to, because a sandbox game like Space Engineers (and also Elite to a lesser extent) depends on the community to the provide living, breathing world that otherwise doesn't exist in the game.
The fellow you quote had a big friend list, so I suspect he feels the same way about Elite that I do about my once great, not so much now Space Engineers server. I also have friends in Elite who no longer play, and
my galaxy is less because of it. Does this mean that the game is objectively dead? Of course not. But it is much less alive for me personally (and everyone else in a similar situation) than it used to be, and that's kinda sad..