T-wall in the bathroom: Drywall stability and construction?

We have planned a T-wall for our bathroom. On the long side (2.20 m), the washbasins will be installed. According to the main contractor contract, a pre-wall is planned. In the plans (2D), only the total wall thickness of 20 cm is indicated. We had a meeting on site today regarding the drywall partitions. When we talked about this T-wall, the site manager said: The wall must be full room height with a thickness of 20 cm due to stability reasons. In addition, it is "only" a drywall partition with metal studs/drywall panels (I hope I understood this correctly, sorry I am not an expert). No mirror cabinets can be mounted on this anyway. We are now a bit surprised, as this sounds somewhat like botched work. Nobody thinks about this when planning a bathroom during the house purchase. Since the space between the washbasins and the wall behind is not very generous, we do not want an additional "pre-wall" in front of the drywall partition. What I do not understand: The area above the washbasins does not have to be as wide anymore, since there are no pipes, etc. running there. Our ideal concept would be: T-wall full room height. Above the washbasin height then a set-back to create a shelf (10 cm). From there to the ceiling, the wall would only be 10 cm thick. We would only mount a mirror and lighting here. Since we already have storage space with the shelf surface. In addition, such a shelf in a T-wall also looks a bit more open. What is the professional opinion here? There is a meeting with the drywall installer. I have already asked what is feasible so that I also have other opinions. Do you see any problems with my plan? Possibly additional costs and from what?