I think its hard to say if TNG would have been better or not if she had stayed. But I have to think that she missed a nice steady paycheck and also 4 movie paychecks. So I think she should have stayed. .
It's easy to say that in hindsight though, when we know the series was very successful, lasted seven seasons, got multiple movies, etc..
But when she made he decision, none of that seemed likely and she wasnt getting much from it, so leaving didnt look like the worse decision you could make.
Denise Crosby left TNG because her 18 hour days involved her standing around on the bridge in the background at tactical and saying the odd line or two. She hated every minute of it and couldn't wait to get out. !
The problem is she was a young actress with no weight the throw around who was given a regular role in a new TV series. She thought she was better and more deserving that she really was. She was also in a contract! It doesn't matter how happy or unhappy she was she was obligated by contract to do a certain level of work. Which, hey, means that her employers were obligated to give her that work or at least pay her a certain amount.
She thought she was bigger than she was, more deserving of something than she was and wrestled out of her contract.
How is that going to look to future employers who see that she couldn't even finish one damn year of a four-year contract before feeling unhappy and wanting out?
If she didn't think the show was going anywhere what would be the harm in sticking it out until the show is canceled?
Backing out of a contract is never a good idea unless you've got an ounce or two of weight to throw around and even then you're asking for trouble. David Caruso weaseled out of his contract for NYPD Blue and, as is the norm, his character was decimated in a way to preclude his return, and Caruso went on to do... Pretty much nothing until CSI Miami came around almost a decade later which revived his career just enough to make him a Internet joke and meme.
Crosby thought she was better than she was when, really, she was lucky to have the work she did have. She was a regular on a TV series with a built-in fanbase and guaranteed at most few years of work in her contract (if the show lasted.) Yeah, she wasn't used much in the first season but neither was Dorn or Burton. Yeah she was told things weren't going to change, which she's an idiot for believing given that even TV producers can't see years into the future. She should have suffered through it or at least finished the first season and then tried to negotiate out of it during the hiatus which would have looked a bit better.
But, nope, she wanted out now! Now! Now! Because I'm a star and I'm special! Well, it worked out great for her didn't it?
As proof that she made the wrong choice just look at what did happen. TNG was wildly successful and got four movies. She possibly could have been part of that if she had just bothered to hang in there for another year or so. (Tasha would've made the transition into Season 3 very well.)
Yeah, hindsight and all but, still, you don't try and weasel out of a contract unless you know something is going to happen for you and you've got weight to toss around. Crosby didn't. She was self-entitled and had delusions of grandeur.