I agree, that the previous comment sounds extreme, there often is a middle ground. People can be reasoned with, otherwise the company you're working for doesn't sound like a good place to be.

Taking a better job is fine, but halfway a project it might kill the project when some of the top guys decide to leave. It might even be worse when the architect of the legacy-system decides to leave with knowledge of all the undocumented business rules.

It's so important, that a project is well managed, because a lot of the aspects of building something new are extremely satisfying to work on. However with mismanagement it's easy to demoralize the team.