There is scant evidence that mods contribute to actual sales. I've seen (via Steam charts) DLC and major content releases move numbers of concurrent players but even then, the pattern for high-vis SP games is massively front-loaded sales and even big DLC a fraction of that. The biggest mod releases, in comparison, were literally invisible.
This is a different game than most. Most games, modding just adds more to the original game but nothing really changes. Warband on the other hand, each total conversion mod changed the game completely, in fact I used to marvel at how the game felt completely different with each new mod I applied. Also most games, even modded won't let you play Game of Thrones, Warhammer, Lord of the Rings, hell Star Wars and many, many other popular setting all in one package.
Personally I was lucky because I got into Warband well after some of the best total conversions were available. If Native would have been the only thing available when I bought Warband, it would have been a 50-100 hour game and never played again. Instead I have several thousand hours on Warband alone. Same for Bannerlord. Totally bored with Native after around 50 hours, no desire to play. With mods, I am approaching 350 hours and can't wait until some of the more ambitious total conversions come online.
I also know that I managed to get like 5 of my friends playing Warband well after it release due to showing them what was available in the mods. There will be tons of people who have never even looked at this game that will be watching Youtube down the road and a total conversion Warhammer, Lord of the Rings or Game of Thrones Mod being showcased and will rush out to buy this game because of it.
As far tracking the influence mods have on sales, yeah I don't doubt that it is almost impossible to track. For example you might get a small uptick in sales when a big, well made Lord of the Rings mod finally releases, but it may take months to make the rounds before everyone who might be interested who doesn't already own the game gets on board. If your tracking this on a spreadsheet, it might not be more than a blip, but in reality 50,000 additional copies were sold over a 6 month period as word of mouth spreads about "this awesome Lord of the Rings game that nobody ever heard about".
I own a lot of games and I mean really a lot. I really like games that can be modded but I´ve never bought a single game because of a mod.
I kind of mention it above but the ENTIRE reason I bought Warband was because I saw a Youtube video showing off one of the Total Conversion Mods, I think it might have been showing off Floris Expanded at the time. I immediately went out and bought the game and have been hooked ever since.
Hell it to a degree even happened with Bannerlord. I stop playing back in May of 2020 after about 250 hours of gameplay. Native wasn't really changing much and the modding community was held up by the constant patches and lack of full modding tools so I got bored. About 2-3 week ago I was just checking out Youtude and saw videos showcasing Ronin and Eagles Rising and said, "Oh hell yeah, that looks cool, time to update Bannerlord and get to playing". I have added 100 hours to my total played specifically because of a Mod.
I don't think mod influence on sales can be visible on a simple sales graph over time. A survey would reveal more.
Mods are almost never released in a classical way, starting with 1.0, they are typically available from some playable alpha state and have iterative releases.
So when Player 1 plays Mod X and has a blast, its version 0.87 he plays. He tells Players 2-4, his gaming buddies, what a blast that mod is, and they eventually succumb. This is not something that happens at once everywhere, so you'll see a sales spike, but is distributed over time.
Probably more influential is Youtuber Alpha who showcases the game with the mod to his followers, who are then persuaded to buy the game for the mod in a large group, which is possibly visible in a sales graph.
It's also reasonable to assume that a mature game with several major and popular mods would be bought by new players specifically for the mod variety.
Yeah this is exactly what I was saying above. It won't show as a sales spike, it shows as sales over time. People bought this game originally because of either the hype or they were people like me who were huge fans of Warband. That is what drove initial sales. You may call it "Early Access" if you want but last March really was release. The actual release won't drastically spike sales nearly as much. What will happen though is that more and better mods will come out once the game releases since there won't be so many patches disrupting the modders work and those mods will drive sales over time. Where many game just have a sales spike at release and nothing more than a trickle after, Bannerlord should honestly see a steady and consistant growth due to how important mods are to the success of this particular title.