Forbes: Employees who stay at a company for more than 2 years on average earn 50% less.

Companies need to get back to rewarding employees for their tenure. Until they do, don't reward companies with your loyalty. When you're worth more on the open market than your company will give you, they are doing you a disservice and you should make the best decision for your own future and go make what you're worth. The more people who do this, the more likely companies will be to change and allow us to stick around and be compensated for it.
Millenials often started their careers in the recession, and have an inset fear of the job market because of it. They often walk around moping as if the economy is stuck in 2009, and it's not. The market is good right now, go test it.