The union’s argument that “our pilots are smart enough to handle two airplane configurations” seems to me to actually be an endorsement of Boeing’s argument that the global fleet benefits from standardization more than a portion of it might benefit from an upgrade.
The other problem is that slapping an EICAS area on the screen doesn’t create the data needed to drive the display. The 737 systems for the most part aren’t computerized, they are more simple and mechanical than newer airframe designs. So, for example, if you wanted to have air conditioning in your remote weekend cabin that doesn’t have electricity, simply buying a window unit doesn’t really solve the problem. There’s a much larger effort needed below the surface.