I think you covered it pretty well in your post below, and as you stated the reasons are compelling. Compelling enough that I am not personally seeing why you are still in favor of all miscues being fouls, although I certainly understand still having a bad taste in your mouth about the way things currently are because it is at best the lesser of two evils.
Regarding #3, a foul being assessed on an otherwise legal shot, simply because there was a human error made by the shooter (a non-foul miscue in this case), just doesn't seem appropriate nor do we do it in the case of any other similar human errors that I can think of that don't otherwise violate any pertinent rules. You are in effect penalizing the human error itself as opposed to penalizing a rule violation. When the shooter hits the wrong ball first, but it is otherwise still a legal shot (happens a lot in 8 ball), we don't assess a foul penalty for that shooter error, so why would we here? When a shooter really butchers their position play, we don't assess a foul penalty for that gross human error, so why would we do it here? Etc. If we are going to penalize this human error, we need to look at penalizing some others as well.
Of biggest concern to me are #'s 4 and 5. I think the rule (if it was effective enough for the intended purposes) could not be written well enough that a lot, and I mean a lot, of subjective judgement would still need to be involved.
The last thing we need is another rule requiring subjective judgement from referees, and even more so when there isn't much compelling need for the rule to begin with. Referees all too often can't even get very clear cut objective things right. A rule requiring a large degree of subjective judgement carries the potential (certainty) for bad calls. Your solution to a problem can't be an even bigger problem than the one it proposed to solve. When being anywhere near on the fence about something, we should lean towards what is going to remove subjectivity rather than towards what is going to add subjectivity.
The last thing we also need are more almost purely subjective things for amateurs to argue about. Half these people can't even tell an obvious double hit foul when there is indisputable objective evidence to look at. I can see amateurs everywhere arguing "that was a miscue/partial miscue, that is a foul!" with the other guy arguing back "are you fricking blind, there was no miscue there!" (or vice versa), and with no way to prove one way or the other, shortly before taking it outside. Yes, it is true that amateurs can use another rule set, but the reality is that in tournaments, leagues, and just while playing on their own, the official WPA rules are very commonly used by amateurs and are often seen as the standard to go to and the standard to aspire to.