Sorry. Tiring week, so I have not really had much opportunity to participate in the thread that I started.
Once you're doing more than reskinning the game, then I think you have to sit down and ask yourself if it still makes sense to work with D&D as the design chassis for this game, or if there are other systems that would achieve what you want to achieve, or if you need to cobble together an entirely new system.
This summation points to the heart of the matter quite well. I will not deny that D&D 5e is overwhelmingly popular, which makes it profitable for 3rd Party Publishers, but it seems as if there are other systems out there that are trying to achieve what some homebrews and 3pp settings/supplements are trying to achieve, but more adroitly. That is one issue. And the other matter pertains to the tipping point of homebrew/designing with 5e where it either becomes another system entirely or exploring other systems becomes the best option. But that may require exploring (1) what "new things" or "new directions" are many extreme end 5e-based 3pp and homebrews trying to get 5e to achieve that were previously not possible, and (2) what are other systems out there with similar design goals and the like.
Likewise, I'm of the opinion that the world really doesn't need a brand new RPG system at this point, and given the great depth of time and effort spent on making rules systems, there probably is no potential rules system out there worth reinventing the wheel over. The history of RPGs are littered with hundreds of fantasy heartbreakers that are often no more than tweaked versions of D&D anyway, and the majority of novel game systems are often inferior to old standbys anyway. After several attempts to create rule systems myself, that resulted in unworkable messes once I added all my cool ideas to them, I've largely given up on that myself. There are maybe a dozen solid RPG systems out there - D20, BRP/Pendragon, D6, DitV, Cortex Plus, WOIN, etc. - that can be tweaked to do pretty much anything you could want to do in any setting you'd want to do it in.
You make some incredibly salient points, Celebrim. I do disagree with your opinion that the "world doesn't really need a brand new RPG system at this point," particularly given how Blades in the Dark (whether suitable for you or not) demonstrates that design space for novel TTRPG systems definitely exists.
So probably the answer is, "Is it actually impossible to tell the sort of stories you want to tell in 5e without tweaking it, and is the amount of effort that would take greater than the amount of tweaks it would take to get a different system to where you'd want it?"
I would favor "practical" over "actually impossible." Is it actually impossible to use a screw driver to drive a nail into a wall? No, but it may not be the most suitable tool for the job, especially if one knows that they have a hammer nearby. Is it possible to use 5e to tell stories of high drama space cowboys? Sure, it's not actually impossible, but it may not be the most practical. Systems like Fate, Scum & Villainy, or even Dogs in the Vinyard may be more suitable.
When it uses both a radically different CharGen and a radically different fortune mechanic or radically different process resolution, then it's probably a different system. If it tweaks only one process in that, it's a different game but it would still be in the D20 system of games - say Mutants and Masterminds or any other True20 game. (Although to be fair, True20 tweaks enough that I could accept the claim it is a different system.) If you make tweaks but you still have classes, levels, hit points, and a fortune in the middle resolution process that depends mostly on the outcome of a D20 plus modifiers, then it's still basically D&D reskinned for a slightly different setting or style of game.
So would you consider the Cypher System as a reskinned D&D game?
Of course, I don't think that really matters too much, as there is no way to measure really how far you've moved from house-ruled X, to brand new game Y, to brand new system Z and largely that doesn't matter except for conveying how familiar a system is going to seem to someone who knows X.
Sure, but that's also why I chose to entitle the thread "the Ship of Theseus," which entails a similar conundrum. We can't really measure when, and if, a ship with new boards becomes another ship, but the underlying question has preoccupied our interest for over two thousand years.
