In particular the brick did not decrease in quality but the way they were built did. For instance for a while people paid less attention to protecting buildings from water damage to achieve more interesting designs.
A particular crazy architectural style that suffers a lot from this is British brutalist architecture.
I have a theory why it's like this. In 1800's architects we're often painters first. Their day job was to paint portraits and signs and whatever was ordered. There was quite lot of painters around, only few of them made "art" and only the best got to draw buildings. The architect would then work the design together with mason. Civil engineers of the time we're busy building railroads.
Then at the end of the century, photography happened. In hindsight it's called "the crisis of art!". Suddenly architects could no longer apprentice by painting stuff for customers. You needed a school for architecture. The teachers would of course be old architects, who hang out with painters. So they sucked that "we can't sell portraits anymore, let's go crazy!" attitude.
In the 1940's you still had some old school guys. During this time some factory owners still thought that paying an architect was investment. You got a factory that would sell your product, keep your employers happy and make you proud. Here is factory building from that period. http://torshammer.eu/wp-content/uploads/2013/01/Kabelfabrike...
Now architecture has not been based on anything for century. Or maybe seeping fashion trends of modern art. Most big money people use architect just to "pretty up" the facade of a building as afterthought. If even that. Nobody trusts architect to make anything coherent or beautiful.
At the same time architects need to jump on any opportunity to get some international fame. Because that's the only way you can ever succeed with such career. This breeds eccentricity. Which makes the whole thing worse.
To justify not using and architect, you might word that as "saving money". To be consistent with that, you cut costs in labor and materials. To the point of making actually bad buildings.
It's a beautiful ruin, but it seems never to have been a properly functioning building. Artistic success, total waste of resources for church that built it.
We are so far apart it is like trying to understand and alien.
I have seen a scant handful of Brutalist building that I consider fine works of craft but the gob smacking majority are pitiful piles of concrete and exposed trusses.