What to answer when Product Owner/Manager says "this feature [requested by users] needs a lot of time to develop, so we’ll never do it"?

More precisely: I work in a company that doesn't have any design culture. Some people tend to think a little more in a user-centered way but not the important ones. The Product Manager only cares about the development time, the CEO only cares about business strategy against competitors, and in the end we are never able to put the expected features (asked for by users) above the stategic & market oriented features (defined by CEO/PM).

Let's take an example: a few months ago it was reported to me that users want to see immediately when a message is read or unread. The proposed solution was: "we have to display in bold the name of the message in the list, and when you read it it turns back to regular, but just for you". But then the Lead Developer said that would need a lot of time to do, because we don't actually know when a message have been read and by whom. Since then, the Product Manager has just delayed the feature, again and again, saying that the impact was too low as compared with the time needed to do it. Ultimately, it even became: "we'll probably never do it at all".

So, as a UX designer, how am I supposed to defend users when these people are taking decisions and just look at me like what I'm saying is useless?