From what little I know about the late history of the Roman Empire, the stirrup thing had nothing to do with it. In late antiquity, Roman soldiers and "barbarian" soldiers were not very different.
Here's an introduction to the scholarly debate about that:
https://acoup.blog/2022/01/14/collections-rome-decline-and-f...
Also, ancient concrete recipes had little to do with it either. Modern construction usually uses steel rebar, which corrodes, because it costs less and lasts long enough. (Most buildings get torn down because they're functionally obsolete.) People who really care about longevity could use stainless steel rebar if they're willing to pay for it. More about that:
https://www.construction-physics.com/p/roman-vs-modern-concr...
Rebar is essential for concrete beams. Something has to provide tensile strength. Roman concrete was usually used in pure compression.
Epoxy-coated rebar turned out to be a dud idea. One scratch in the epoxy, or a cut end, or a weld, lets water in. It's now banned in Quebec and being looked at elsewhere.
The Oregon Department of Transportation has been using stainless steel rebar for bridges that cross salt water. Seems to be working out well. Steel cost is 5x-6x ordinary rebar, but the long life is worth it.
There's now stainless steel plated rebar, which is a tougher coating than epoxy. Not clear how that will work out.
What features of modern construction create tensile stresses where the Roman designs only produced compressive stresses?
You'll notice the Romans used a lot of arches and very few concrete beams.
Introduction to bridge statics [1]
[1] https://www.youtube.com/watch?v=SbCVRr5eANA&