No one's talking about building ethically-problematic software projects here like software to aid despotic regimes, harm human rights, etc. We're talking about business projects that senior engineers believe will fail either in execution or in the market. There's nothing unethical about just letting these things fail, especially if they aren't even a project you're assigned to work on. It's upper management's responsibility to assess risk and pick good project to assign resources to; as a senior engineer, your only job is to advise them when consulted. If upper management is incompetent and doing a bad job choosing projects, then your recourse is to go looking for a better-run company, not telling your executives how to do their jobs.