First and foremost, remember that you're writing code to bring in customers. Your codebase can be beautiful, pragmatic, semantic, and have 100% test coverage; if you don't have any customers, you don't have anything.
"Unnecessary complexity to the codebase"
It depends on what you mean by unnecessary. If you mean "won't bring in anymore customers", have that conversation with your managers. Not all of them are brilliant, and no one gets it right 100% of the time. If you can prove that the feature doesn't provide value, have that conversation with them.
On the other hand, if your boss ignores your input, and you're 1000% sure that there are other features that are more valuable to your business than the one in question, you can always push that one to the back and work on something that's more productive to the company. Depending on your political and professional circumstances, your boss may not notice or care, and their boss may forget about their red herring feature; you might be able to side-step the conversation altogether. This will only work if there's more than a few items on your plate that need to get done soon, and this feature can get pushed aside without delaying or blocking anyone else.
Bear in mind that if you go this route, you're putting yourself, your career, and your neck on the line. If it turns out that it wasn't a good idea and everyone agree with you, you'll look like a genius and gain some clout as a clairvoyant; if it turns out it was seriously necessary, you'll look like someone who pouts when they don't get their way. Either way your boss may also hold a grudge. I'm not saying it's the greatest way to go, just adding it as an option. It's helped me more than a few times in my career, but it's also frustrated my bosses a few times. Be gracious if you're shown wrong though, and quick to admit defeat if it's obvious you chose the wrong path, and you should be fine no matter what happens.