That sounds like a bold statement. I would imagine the large majority of players of any mass-market title don’t know about or use any non-built-in mods.
That is almost certainly a vast majority of PC users using that single mod! This is also probably why Bethesda tried to make 'paid mods' a thing, and bring them over to consoles. Not only does PC seem to be their best selling platform, but people are largely buying Skyrim to mod it, and Bethesda wanted to try to start taking a cut of it.
On the other hand I'd completely agree with you for games where modding isn't so well supported. That results in more technical issues, and less impressive mods. I mean in games like Skyrims you have literally entirely new and complete games built as 'mods.' Some even get their own independent releases, like Enderal [2], which many would claim is [vastly] better than Skyrim itself.
[1] - https://www.nexusmods.com/skyrim/mods/3863
[2] - https://store.steampowered.com/app/933480/Enderal_Forgotten_...
So in a best case, it's 50% of PC players (which may apply to steam deck), or worst case, less than 15% of all players (which may also apply to steam deck).
[1] - https://www.gamesradar.com/skyrim-has-sold-60-million-copies...
I'm only talking about "Skyrim", the original game. It sold in the ballpark of ~25 million, with a probable plurality on PC - so somewhere in the ballpark of 10 million there. And no, like I mentioned, the download numbers are unique users - not total downloads. The total modding userbase is also going to be well upwards of 8 million. There are lots of reasonably sized non-English Skyrim modding sites also hosting SkyUI, and there's probably at least a small number of modders that have never installed that specific mod.
I'm quite confident on the PC sales, because there's an oddly consistent little metric. For games that sell beyond a minimum amount, about 1 in 60 players tends to leave a written review. So total sales tends to be in the ballpark of written_reviews * 60. For Skyrim, that's 180,000 reviews or 10.8 million sales [1]. Linked because the game's been unlisted since the Special Edition came out. That metric also matches the mod count near perfectly, as well as Bethesda's sales announcements, and also their behaviors like trying to do things like create 'paid mods' (and the predictable backlash it entailed). So I'd estimate reasonable figures would be around 10 million on the PC, and a 2:1 360:PS3 ratio for the remainder. So you end up with somewhere around 10 million, 7 million, 4 million, with a variance probably in the range of something like 30%.
[1] - https://store.steampowered.com/app/72850/The_Elder_Scrolls_V...
Sure, there are console players that have no means of installing a mod, but I'd be shocked if even 10% of PC players didn't use at least a couple.
After being just about finished with the game, I decided to try some mods. Things that should address pain points, like indeed not actually being able to use most of the map, and traffic management.
The traffic turned into so much micromanagement to get cars to pick the right lane, and every time you made a change anywhere it required updating half a dozen other intersections as well, it just wasn't fun to grow the city bigger than possible in vanilla.
I remember one of the last games being an attempt at public transport only, where housing and shopping and such were accessible only by taking the subway. That worked super well in some aspects, and it's fun to design the mass transportation systems to underpin that city, but Skylines is so much geared towards vehicles that I didn't feel this was a proper city either. (From what I remember of a single game I played five years ago.)
Anyway, what I'm trying to say: based on my own and my girlfriend's experience, not everyone plays with memory-hungry mods, or any mods at all