This is me being a bit pedantic but what you describe is not Sunk Cost Fallacy. You are describing a decision to invest in a less enjoyable game because it has a stable fan base so there will be more chance of finding players.
In this context, Sunk Cost Fallacy would be when gamers continue to invest their time and money in Warhammer because they are worried that if they stopped doing this, their past investments of time and money would have been a waste.
That said, I definitely think the Sunk Cost Fallacy applies and is one of the reasons why some wargamers persist with wargaming even when it no longer brings them joy.
But that's slightly different from reasoning that you want to play a certain system cos you know it's gonna have enough players to get games regularly.