Any mature development manager would understand turnover is normal. They would also try and convince you to stay until the launch. If I were you, I'd just tell

Any mature development manager would understand turnover is normal. They would also try and convince you to stay until the launch. If I were you, I'd just tell some convenient white lie that gives you a good out without burning bridges. Your top priority should always be you, not the company nor your co-workers. Do what's best for you.