For the record, I'm not involved in HOW our publisher handles games on the forum anymore.... But, a quick check shows that, Darkest Hour was 101st places amongst Paradox products last month in revenue, and 84th total in history.
If I would run a business, I would expect that such a title would be moved down when it gets over a year old, and there is no planned new revenue for the franchise.
But revenue = price * quantity, and profit = price*quantity - cost*quantity. Marginal Profit is likely close to price due to the fact that selling another copy of a software in general has near 0 MC (some server costs of course). Using revenue as the sole comparison for a game that sells for $10 or less ($2.5 at sales) compared to other games which sell for $50, 60 at launch (well, Paradox games usually go for around $30-40) is an unfair comparison, to say the least that you are ignoring high margins on each sale.
I won't dispute that you likely make more money from promoting a recent game though which sells for $30-60. Good business principle dictates that there is an opportunity cost to every action, no matter how profitable it may be, and you are probably making a good business decision.
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Business aside, I don't know how much PI forums drive sales to be honest. Forumites, as was noted in some other threads before (I remember it was in one about Steam), do not make up a majority of purchases these days. I think PI forums provides a nice community where people who enjoy your games meet and provide "extra-value" for the consumer via mods and tech support while also helping the producer (you get feedback and free community support, although I think the consumer benefit from the forums may be higher). At the end of the day, I feel this is a more important factor and that active communities, even if old, should be given more of an equal footing versus not so active but newer games. Currently, I feel the forum unfairly creates the notion that "recent" games are active and new, while everything else is near the end of their product support line/"dead". I think part of the reason (I may be putting words into his mouth though) that others in the team were unhappy over the decision is due to this, as the notion that the game is "dead" and no longer going to be supported undervalues the continued time and dedication the team puts into new patches. Of course, this isn't the forum's fault. Realistic truths about how long a game is supported after release probably contributes more to this. There isn't a really good solution to it though.
The forums is already split into PDS and non-PDS games as it is. Imo, I liked the forums better before that split was made, but that's just my own two cents. Of course, you have more data and experience than me, so feel free to correct my purely theoretical and unsupported hypothesis, and/or opinions.