So you have no link showing "streamers" winning the game, all your arguing on difficulty is crumbling.
But for sure experts and amateurs have hardly the same perception, for games and difficulty but also art and entertainment.
EDIT: Your quote on campaign aspect is showing your total ignorance of turn based games, it's nothing new that campaign level is used to define and control the difficulty feeling. It's a long topic, but a different topic, but one more topic related to difficulty perception.
EDIT2: Shift of difficulty on campaign level is nothing new, but there's probably a tendency to increase the shift. Yes not many game will allow a perfect campaign play without skills to predict future or skills to read mind of dev. But for games based on a design with many replays that is typically Roguelite/Roguelike, the problem is less obvious. The benefit is when the campaign level is a bit screwed up it increases tactical combats difficulty, and then helps not feel the game is too easy and straight forward. Some player tools, not lame for many players, allow tune difficulty at campaign level, a typical example is grinding. Another example is global management flexibility allowing fix errors. I'm not fan of it but I think tactical combats benefit of added layout.
Darkest Dungeon clearly exploit it, it's not just a combat, it's series of combats inside an additional layout, expedition management. There's also another layout that is the general management/campaign level, for example end a dungeon with many heroes heavily stressed is a complexity element to consider in combats even if it's not directly related to combats.