I disagree. As far as I know the alex fork of source 2 (which is what s&box using) did follow the map limitation in source 1, but the engine itself did actually could support a larger map size (Iirc l4d3 is going to be a open world shooter, so there must be something related to open world stuff still in s2, like level streaming and stuff, alyx build did get a incomplete implementation ).
Plus, making open world stuff is definitely something hard, but not rocket science nor require a recode due to the reason I mentioned above.
Regarding the userbase, instead of saying “this is not worthy to make cuz it only appeals a small userbase”, but let’s say it is because there isn’t a proper support on large levels, so the developers have to chose other platforms, like unreal engine. A large map support would definitely appeal the RP community, I mean who wouldn’t mind to have a larger map for a better, more immersive role playing experience. In short, adding large terrain support could allow the game to expand its target customers/player base to a larger number.
Unreal engine is excellent in large terrain games, but meh in modding. Gmod is excellent in modding support but large terrain support is non existent. I hope s&box could adopt the advantages from both worlds.
P.s: for dependencies, I think garry could make an automatic packaging tools like how web developers had(webpack, snowpack or vite), which helps modders to package all the required assets or codes into the addon/gamemode. No one likes missing textures, but no one also likes pasting code by themselves