On the basis of these two, quit and see what else is out there. Perhaps the new job will be amazing, it might be crap. Either way it will give you some more perspective on what things are like out there. Your history is 8 years in the same company, if you are so valued, they will take you back if a position opens up.
The fact they gave you a poor pay rise recently, well its difficult to say how your company has handled the past year, and everyone might be in the same boat.
The second paragraph, you said that they indicated you are a key player and only expert in the product, but contradicted that by saying you could easily be replaced.
I was in a similar situation a few years back. I was actually in the same job for almost 10 years, that I started as my first permanent job about 6 months after finishing University.
When I was new, the company seemed huge, and there was a lot of potential to progress. As I developed, I started to realise that I was becoming more expensive for the company as I progressed, less and less doors opened up. Not just because I was working my way up the company, but because the higher up you got, the less people moved about. At first I thought this was normal. I was told that I was one of their UK experts in my field, and played a key role in our European OPS Model Governance Forum, helping decide the future direction and controls for our area within EMEA. I actually found out that I had very little say, or control over changes and it was just a fad.
You are better to live your life and say you took some risks, rather than regret decisions you didnt make. Some will go better than others, but end of the day you will progress and develop as a result.