There's an argument that in polyamory, you can't reach the same depth as in monoamory.

Just as with a similar argument about love, this seems to view "depth" as one finite element.

If you look more closely at what people place in "depth", you will often see a multitude of elements, and you will also often see that not all of those elements will be part of one relationship. Some of those elements will not come up before the right person to share them with comes along.

So what if, in a polyamorous relationship, person A shares elements x and y with person B and elements z and t with person C. Does that mean less depth than if person A had first had a relationship with person B (sharing elements x and y), and later on had a relationship with person B (sharing elements z and t)? And even if there are some elements of "depth" shared with more than one person, does that have to mean that less of that element is shared with each person (thus following the idea of a finite element again)? And if that element was shared with just one person, would it really automatically be shared more in depth with that one?

This is another argument I call a myth. I base this mainly on my own experience, where I can see just as much depth growing in my current two relationships as I saw with Eva, one of the deepest monoamorous relationships I've been in.