I think it depends. Is the marriage unhappy because the partner are poly and can't be themselves as long as the marriage stays monogamous? Then yes, poly's going to solve that problem.
In my case, poly was good for me, bad for my husband. I thought it helped both of us and the relationship, but after breaking things off I was told that it actually didn't.
I won't have this problem anymore, I'm starting off poly now, but I see how it could end badly much more easily than end up positively. It's a bit like having a kid: if you were having issues in your relationship BECAUSE you both wanted a kid and were having trouble having one, then yes, having a kid with solve that. Otherwise, you're probably better trying to solve your problems first, rather than have to do it when you have to deal with the stress an extra person can bring.