@Daniel B.
It sounds like you had a bad experience with your architect due to a misalignment of project goals. This could be due to a number of factors the most likely being a lack of communication.
Generally speaking, architects understand that stacking plumbing walls is a best practice, and adding steel to any wood framed residential project is an added expense. Typically an architect would not add steel to a simple home unless the plan that was reached made it absolutely necessary. If you and your architect had been on the same page with budget being the most important variable to consider they likely would have approached the framing differently. I clearly wasn't involved in the process but this is my suspicion.
Your comment regarding the use of engineered floor joists is both correct, and incorrect. Housing trends have largely moved to more open plans with less interior walls and intermediate support columns, an efficient way to span these open spaces is via the use of engineered floor joists. While the joists do cost more than traditional lumber they have largely become the standard for framing floors in residential construction due to their increased stiffness aka. their ability to reduce deflection in the floor, in general less bouncy floors are preferred by users of all building types.
I thought it would also be best to mention that wood floors will not do well in a fire regardless of whether you use 2x lumber or an engineered joist. If you want floors that will handle a fire or allow for one unit to go up and have any chance of saving the adjacent unit you will need to look into building fire rated assemblies where the majority of their resistance to catching fire is based on the materials that face the framing ie. gypsum board.
An architect isn't the right choice for every project but a good one can help find efficiencies that will save you money on the build, help get you to a better finished product, and more than make up for the small fees we charge (usually well under 10% of the total development cost). If you aren't intending to build a high quality product or you have the experience to understand their shortcomings and work around them than stick with builder plans.