Thank you everyone for your replies. I am not against a simple and modest design of Catholic Churches and have been to mass at several beautiful and very simple churches around the world. Also sometimes the extremely ornate baroque style in some European churches can have the affect of being somewhat triumphant and lacking in modesty IMHO as a convert from aethism (especially when they are now empty).A harmony and balance can be found in all things.
What I am describing is probably a reflection of the lack of fervor within the church (especially in the leadership) in Germany where a lack of catechisis also means parish members no longer understand what is missing. It is a spiritual starkness. I also agree it does reflect the architectural fashions of the 20th century, especially post-war. I am also aware it happened in many countries, especially in Northern Europe and the USA. I was just wondering if there is evidence that German theologians were at the vanguard of this change as someone I met claimed.
The German church is extremely wealthy due to automatic taxes (effectively a tithe) that all Catholics pay. So Bishops have the money to make sweeping changes.
Sadly the church is in major and massive decline there. The church in America is absolutely thriving in comparison. As I said, there is hardly anyone under 60 in the churches (at least in Rhineland-Pfalz where we were). Our young family with children was highly unusual. Any of the very few young children were brought by their grandparents rather than their parents if they were there at all. When this older generation (baby boomers) dies out the church will be in an even worse crisis than it is currently.
I think catechisis, or lack of it had much more of an impact than WW 2, the Baby Boomers in Germany were obviously born after the war and they seem to be the last well catechized generation who still attend mass in significant numbers.
When we arrived in Germany from the USA and I asked a priest about what was available for my children to help with their catechisis, he actually laughed at me and became extremely annoyed. He told me the schools took care of that! These are secular, public schools. I wanted to say “and how is that working for you?” because obviously none of them were coming to church or their parents. I also met some very holy priests and parishioners but they are a tiny percentage of a large baptised population that do not practice.