I've been a WWII flight sim fan since Aces of the Pacific came out in 1992, and my observation is that the most successful sims have two things in common. They all received great reviews from reviewers like PC Gamer at some point, and had a certain amount of mass appeal. I believe these are two of the secrets to gaining the wider following and commercial success that these games enjoyed.
The sims I'm referring to are Microsoft Combat Flight Simulator in 1998, and IL-2 Sturmovik in 2001. Both of these sims featured in PC Gamer's Top 100, and I personally purchased both on the basis of this recommendation (though I didn't buy into IL-2 until Pacific Fighters came out in 2004). I don't think any WWII flight sim has achieved such a high recommendation since. I don't know
what the devs did to achieve this (sweet talk, bribe, salesmanship or just design brilliance), but I think any aspiring devs these days need to get those reviewers on side
somehow. This would ideally include having a polished product on initial release which is too good for reviewers to ignore, including an easy arcade-style entry point to appeal to casual gamers while still being a fairly hardcore sim for the serious flyers, and not doing silly things to annoy the player.
Or if the devs of an existing sim that has been refined over the years,
like Great Battles, think their sim is now genuinely better than "1946", "DCS" or "War Thunder", it's probably never too late to actively seek some fresh reviewer recognition and promotion of it.
As for subscription, that might work for the lemming types who don't seem to think too much about the cost as long as they're playing the latest fad game, and it might work for hardcore cashed up fans, but I think most combat sim fans of the type that bought into those previously successful titles are more discerning than that.