The key as I see it is that in opera the theatre is the singing. It all hinges upon on a very specific utilization of the voice married to music and connected to an open person who is willing to let their soul be delved into and have it laid bare in their singing.
If someone goes to the ballet, would it be wrong to say that ballet is about dancing? If someone is drawn to the symphonic playing, there are other mediums for that. If someone is drawn to the theatricality, there is straight theatre, musicals, etc. It is the dancing that separates the art form from other genres. It is a highly sophisticated form of movement, and if that isn't present the art form is not being served properly. Like operas, those works were written for dancers who could express themselves within the confines of the aesthetic tradition of balletic movement. No amount of emotional expression on the part of the dancers or clever direction or a production can make up for that, because that's not what the art form hinges upon. If you can't move your body a particular way, you may be a wonderful performer, but not a ballet dancer.
In opera, there is a visual element, there is an orchestral element and most importantly there is a vocal element. The stories were conceived to be told via human voices in unamplified environments, and if one's voice is limited in what it can do acoustically, one is simply a deficient operatic performer. The singing is central to the cohesion of the enterprise. A production can give direction to the story telling and help the singers access more sincere readings of the emotional content of their vocal lines and the text, but physical manifestations of that are absolutely subsidiary to what is expressed musically. Most of the most passionate, ecstatic mooments in opera are scored in ways making comprehension of the text impossible, because it was meant to be a sonic experience wherein the truth was conveyed through the sheer sound of the human voice filling up a space. If not, why aren't Judi Dench, Helen Mirren and Meryl Streep (some of the finest actresses of our time) famous Cio-Cio San's? Because they can't sing resonant Bbs over an orchestra.
Opera is musical expression through singing.
Period.