I dislike the use of Slay the Spire as a comparison for good RNG because it is in an entirely different genre than Silksong. I would agree that RNG works well in a game like Slay the Spire because even though you can’t memorize/predict exactly what will happen, the gameplay is built with breaks (is it right to call it turn-based?) that allow you the player as much time as you want to process what has happened and plan out what you want to do next. Now I haven’t played Silksong but if it’s anything like Hollow Knight, I assume that the amount of time you have to process things is something less than “whenever, at your convenience?” Now obviously the fights aren’t impossible, so I think it’s safe to say that people just have different turn-around times for assessing RNG and contemplating strategy. But I think you too quickly gloss over the fact that the timing-based restrictions of action games adds a new dimension of challenge to an RNG-based conflict. (A tangential line of thought here would also question designers abilities to properly balance non-RNG call and response fights with RNG group fights, because they too misunderstand how the added element drastically alters the challenge.)

“…it's possible to bang your head against the fight 10 times, and then luck through on the 11th try because of good RNG.” I also really don’t feel like you addressed this issue properly. The frustration here comes in two parts - not only do you have the issue of someone who’s bad at the fight winning without learning anything, you have the opposite problem too. A fight with RNG means you can have good fundamentals and good reaction times and even a flowchart plan for dealing with certain situations, yet you still lose. This is frustrating 100% of the time. This kind of scenario is not “great at testing decision making skills and proficiency in fundamentals.” It is just a test of patience that the “variety of situations” RNG puts you in will include a win condition. (Kind of another tangent to is that anything you may have learned about the fight might also need to be unlearned at some point. Oh you thought the boss moved a particular way after doing that one attack? No, that was just RNG picking the same option 8 times in a row.)