My doubt about @Eagle Pursuit statement was only in regards to his assertion that the proportion of high skilled players is shrinking
Honestly, it seems impossible to know. It's easy to shake one's hand at the youth these days not being as committed to strategy games as they used to be, something I see a frustrating amount on the internet. As you've already discussed, it's so dependent on what meanings people are bringing to this. Purely by achievements on Steam you could make the opposite argument; 82.3% of players have founded a second city, something about as hard as the achievements we used previously as a benchmark for "has booted up the game" compared to ~75% for our Civ 6 benchmark, and only 3.1% of people have beaten Deity to Civ 6's 6.4%. You could plausibly argue we have an increasingly large proportion of the fan base beating Deity; you could also argue Deity is harder in Civ 5, or it has been out for long enough that a large chunk of people to buy it for a few bucks and not really engage with it, or any number of arguments here really. No-one knows what proportion of the user base is highly skilled players; I would say that a larger proportion of the casual fan base is able to discuss the game now than when CivFanatics was founded. If you were talking about Civ on the internet in 2001, you were probably pretty committed - nowadays, someone might post on the Civ reddit the first day they get the game to try and answer a question. That can look a lot like an increasingly casual user base, even if it's just increased visibility of those players.