There are a few ways I can see this working out.
Display is essentially a synonym for screen, and a monitor is a type of screen, so monitor is already just a subset of display. So should we get rid of monitor? I say no. A monitor is a specific piece of hardware used to see and interact with a computer. This makes it an essential tag to the site. display covers a much wider range of screens that could be on anything from a computer's fan controller to a smartwatch — I wouldn't necessarily call these monitors.
So to merge monitor → display, you would have to argue that monitors themselves aren't a major piece of hardware this site focuses on.
If you wanted to merge display → monitor, you would have to argue that a monitor is absolutely any type of screen on any kind of hardware. This is a big stretch in my opinion.
I think we should keep them as is because they both refer to certain things. However, if we do this, we need to work on cleaning them up and clarifying their meanings because we wouldn't be here if they were already like that.
Based on the definitions I gave for each tag above (in the first paragraph), I have come up with simpler definitions:
monitors — any standalone screen that isn't physically a part of another component. The obvious example is a computer monitor you connect to a computer.
displays — any screen integrated into a device. Good examples are kiosk screens and e-reader screens.