"If the fool would persist in his folly he would become wise".William Blake
The product version of this wisdom - if the product manager would persist in his bad decision, he would fail and learn from it, hopefully. But if you save him from the failure, he won't learn and will come up with more bad decisions.
Let's admit, most of us learn from our own mistakes. The best of us are able to learn from the mistakes of others. And hopefully none of us won't learn at all. You can't stay a PM for a while if you don't learn at all. But what if you see someone making a mistake, but you can't convince them to reconsider? Especially if that someone is your superior.
By definition, we, PMs, are an opinionated bunch. We're expected to be the experts on our products, we are expected to be data-informed, and we are expected to have a "product sense". That last one is often hard to quantify, and it therefore could cause issues for us and our organisations.
Imagine a typical situation from the life of a PM. You get into a meeting where a decision is discussed. You are prepared, you present data, you talk about strategy, you show beautiful slides... Then your boss starts to speak and basically overrides everything you just said. They don't have data, they don't link back to strategy, they didn't even prepare a single slide... But it's their idea that everyone agrees with. You don't, you try to fight, but you're outnumbered and outvoted. You have no other choice than to disagree and commit. You leave this meeting all but sure that the chosen idea is wrong and that it would fail.
When this happens to you, there are at least two paths you can take. You can sabotage the idea and prepare your "I told you so" face. Or you can do whatever you can to make that idea work. Maybe if you put an extraordinary effort, you can make this idea work. But if you do so - your boss won't learn, and next time you'd have yet another bad idea to deal with. What will you do?
If I were answering this question early in my career, I'd definitely say that you should try to make the idea work. Besides, I would have added, if you make the idea work, your boss will notice your amazing efforts and reward them accordingly. And then I would be very puzzled why the number of bad ideas would increase, until I'd basically be bouncing between one bad idea after another until the total burnout.
What I learnt the hard way - sometimes you need to let bad ideas fail so people can learn. Yes, at times you can salvage a bad idea with extraordinary effort, but longer term, it could backfire as people who needed to learn from their mistakes won't do it. Sometimes you need to live and let fail.
Does that mean you should sabotage bad ideas? No! You should stay professional and do your job at your consistently high yet sustainable level. Don't go to overdrive mode to save a bad idea, as it will backfire in the long term.
But, some of you might say, if the bad idea worked out because of extraordinary efforts, does it mean it was worth it? No. Because of the opportunity cost. Bad idea + extraordinary efforts = average result. Good idea + extraordinary efforts = exceptional outcome. Every bad idea you pursue is a good idea that is left behind. That's why a good PM would be equally proud about things they didn't do because those were bad ideas.
And finally, what if you thought something was a bad idea but it actually turned out to be a good idea? Well, that's your learning moment. You should let it in humbly and get the most out of it. Think: what signals I misinterpret? What additional information should I have sought? Whom should I have consulted? Could I have failed faster? Cheaper? Who was right? What did they know that I didn't?
Bad decisions will happen. Failure's inevitable, that's how we learn. What we should be trying to do is help each other to fail faster and less badly. We should help each other learn by letting each other fail when we cannot be convinced to learn from the failures of others.
Live and let bad ideas die.