My comment about thinking shutting down once a child had been spoon fed an algorithm comes from what I experienced teaching third and fourth grade.
I would have (average) students, who were just beginning to really develop some number sense and mathematical understanding, and then they would go home and their parents would teach them to borrow and carry. The kid would come back to school and be totally confused, grasping onto the algorithm like a life raft and unable to think any deeper.
As a teacher I found that it was often the parents with the weakest mathematical thinking themselves, that are so passionately resistant to Constructivism. In turn their children were the ones who would be still trying to cross out the one in problems like "101-99 = ?" The children of engineers, mathers or scientists usually had a leg up, because they had been talking about numbers in a global sense, since they were very little.
Regarding Roman numerals... Yes, sometimes it is easier to just teach something directly. As with any educational philosophy, you can't be too hard-core all of the time. But on the whole, I think it is best to lean into children making learning discoveries for themselves, with you as the teacher or parent providing lots of structure and guidance to make this happen.
