O.K. first an admission, I am American, and know we are disliked here but I am going to throw two cents worth in here anyway.
For the management from a purely cost viewpoint combining up numerous approach controls combined up into the same building makes tremendous economic sense. All approach controls will have numerous pieces of equipment, interphone communications lines, scopes, computers, etc. If all are grouped together there is less maintenance staff needed. Fewer buildings to maintain. Theoretically a more streamlined training program. Easier and cheaper to transfer between facilities or different areas. Also allows lower pay for the tower only people as a separate entity than would be normal under a combined traffic count profile. May allow for combining functions and a reduction in required manpower. These are all tangibles that are easy to quantify.
From the controller perspective there are many intangibles that are extremely important for an efficient movement of traffic. All controllers know that it is beneficial to have intricate, in depth knowlege of what others are doing an why they are doing it. This would normally occur in a breakroom or an afterwork type of establishment. This interaction is important for all concerned but it is an intangible that is difficult to quantify. We have seen numerous times that the split of a Tower/Tracon into separate facilities will eliminate this interaction. It makes no difference is the two facilities are still located on the airport, but in separate buildings, or if they are many miles apart.
Once the interaction ends the passing of knowlege will also end. In a fairly short time, those that have the knowlege from their pre-split days will forget all about it and the animosity between the two facilities will begin. It will become a downward spiral with essentially no end.
One side no longer has any idea of how they might be hurting the other side, and no longer care. It soon degenerates to telling on each other, which leads rapidly to CYA being more important to moving the traffic.
The bigger/busier the airport/approach the faster it will happen.
This might explain why the previous Americans advised not to do it. Unfortunately, you organization will only focus on the the costs and tangible evidence just like ours did. The end result will work, just not as well as it used to.