I agree that coaching is not really the issue here with Novak, but I do think it can help all players at certain points in their career, even the greats. Sometimes having a different voice after many years is helpful. I think Ljubicic and Moya may deserve more credit than they get for Roger and Rafa's resurrections.
Indeed, they both do.
But let's look at what they've really done (and that is not to downplay their success, but rather to put it in perspective): Roger is more aggressive on second serve returns, has gone back to taking the ball consistently early and ending points quickly. Rafa is serving better (especially in 2017, not enough of a sample this year to judge), using his slider out wide more, occasionally stepping inside more on the return of serve, and generally being more aggressive from both wings, going for his inside out and DTL forehands more...
These are not huge adjustments (the effect they had were huge though). This is pretty much what everyone and their mother notes when watching them struggle. The reason I bring this up is, when it comes to great players, you seldom need a coach with a bunch of creative ideas who attempts big changes. In fact, I'd argue that Higueras, who tried to change too much in Roger's game (way too many drop shots on clay, and an overall kamikaze approach which backfired disastrously) and even Edberg (who had some success) tried to implement way too many changes in Roger's approach. We're talking about the most talented player ever and probably the most well-rounded. There isn't much that needs changing in his game. You don't need to come up with some gimmicky insane tactic with a cool name that looks awesome when it works but was by no means sustainable (SABR. Sorry, but as amazing as it is, half-volleying second serves from almost inside the service box is probably not the answer to a man's struggles).
Likewise, you don't need someone flattening out Nadal's groundies, or make him play an overly aggressive game that he's just never comfortable playing.
In both Fed and Rafa's cases, you just needed someone that A) helped them play their games as effectively as possible, B) help in very specific areas in which they had struggled with. Both Moya and Ljubicic were able to do that, and that's what sets them apart. I'll also give both credit for being able to establish a rapport with the player (though in Moya's case that wasn't difficult due to his friendship with Nadal), and finding a way to establish mutual respect and belief, which is often the most difficult part in a player/coach relationship.
The reason Toni Nadal and Rafa are the most successful player/coach combo in history isn't because Toni is the best coach in the world (he probably wouldn't be half as successful with another player). It's because with Rafa's well-documented doubts and insecurities, you needed a coach who truly understood them and managed to push Rafa to step outside his comfort zone (let's no forget how limited his game was early on) in-spite of those insecurities. I don't think any other coach could have done that because they simply wouldn't know Rafa like Toni does, and Rafa wouldn't "let them in" the way he would with his lifetime coach and uncle. That's why personalities are often more important than ideas when it comes to coaching in tennis. Nadal is a private guy. He wouldn't just let anyone into his entourage. That's a big reason why the Moya thing worked out so well. He's one of his best friends and like his uncle, knew him since he was a kid. In many ways he's also a mentor.
If you read some of Moya's comments, you can clearly see this is a guy who understands Rafa. I remember reading him say that Nadal is usually much more aggressive in practice and that he sometimes wishes Rafa brings the same attitude to the court more consistently. But I think Moya understands that this is just not Rafa's nature and he wouldn't push him too extremely outside his comfort zone, something that another coach might have attempted. Another coach, knowing that Nadal is theoretically capable of playing this way would probably not see why a player this good can't do this in matches, and might have pushed Nadal to play a game he's just not comfortable with, leading to some poor results in which Nadal is unsure in his approach.
And that's why so many of these high profile player/coach combinations don't work out. Often times, the coach is attempting to do too much because they are coaching a very talented player. I actually think that's an issue many coaches had with Federer (he can do it all, so they feel like they can try it all), and I have a feeling that's what happened with Agassi and Novak. I suspect Agassi's ideas were too extreme, and that in his mind, he didn't see any reason why Novak wouldn't be able to play the way he wants him to play, given how talented he is. A lot of these coaches have certain ideas and visions that are probably sound in their own heads, and think that just because they're going to coach a guy so good, that everything will work out fine. I mean, who can forget Todd Martin messing up Novak's serve almost beyond repair?
With these great players, you typically want something far less extreme.