I think the problem is when you start trying to get away with doing less than what you're paid for. For example, taking short cuts, hiding issues, or neglecting other responsibilities. In many roles, this would probably work for quite a long time but create massive long-term issues for the company.
Does that matter if the company allows the culture of 18 months then new job?
It's the company's job to create a long term culture. If anything people who can hide bad stuff for 18 months then get a 30% pay raise are simply optimizing for the reward mechanisms in front of them.
