<<Where is the line to be drawn?>>
I take your point. The Council speaks of reforming the liturgy guided by the principle of "noble simplicity". Clearly people can disagree about the application of the principle. I think, though, that it is not difficult to set out some markers at the extremes. When we celebrate the liturgy, we are doing something noble and sacred; liturgical vesture ought to reflect that.  "Noble" does not mean ornate or gaudy; it certainly does not mean triumphalist.  "Simplicity" does not mean rags.  Thus under the rubric of "noble simplicity", I think it is easy to rule out the extemes: no torn jeans and tee shirts on one extreme, no cappa magna on the other.
The cappa is not even properly a liturgical vestment. The celebrant puts it on precisely to take it off; but putting it on and taking it off is as much a spectacle as is the garment itself. In liturgy, we worship one who came to serve, not to be served. The cappa is, at best, a distraction.