Required, important, irreplaceable, necessary, integral.

There are lots of ways to express an idea besides this one trendy construction metaphor

It is really trendy? I don't think important, irreplaceable or integral are semantically equivalent. Required and necessary almost get there.

The point is that if you want to remove it or change it, you need to put something else in place that handles the work it is doing. So it is more that it is serving a required/necessary function, without being any of those other things in and of itself. It's more about "be careful when changing this."