God says this to Moses to halt all the complaining the Israelites were doing against Moses and his leadership. They wanted to be in charge. They thought they could do a better job most likely. I'm sure you've been on one of these sides -- maybe both. I remember speaking negatively about a leader in my life. It was wrong. And it accomplished nothing but personal discontent. It surely didn't make the situation better.
I've also been on the receiving end of grumblings. And the natural tendency for us humans is to try and convince those who are complaining that we're okay as we are. This is almost always a waste of time.
We need to let God deal with it. If we try to justify ourselves we can fall headlong into a trap that we can't get out of.
Leave it to God.