I don't see it on the decline. I see it replaced by mistresses, muta'a, and I cannot remember the Sunni term, nearly no one talks about it. I think if you are going to marry a Muslim man you had better accept that they don't have the same vows you do. They don't have to be faithful, and they know it. Sorry if this is harsh, it was true for me, and many women I knew. Men just don't see cheating as a huge deal when they can have multiple partners, how could they think of their own sexual fidelity as a sacred trust, when it is not expected of them?
Aha...I don't know why it seems that way to me. I guess among the people I've spoken to, there's a lot who are starting to view men who have multiple wives without a "valid" reason like infertility as kind of sleazy, and more people becoming interested in love marriages. Among my in-laws, two men got second wives, I believe around a decade ago, "just to have fun," or so I've heard, and I am under the impression the new marriages were not well received and that there was a lot of disapproval at the time...
I see what you mean, but I'm hopeful that it will start to fall out of favor in time either way...