My parents still handle easier the UI where they see the "buttons" for the navigation. They are just confused when some of the text is clickable and some is not.

For those who don't know them, the "buttons" like "Cancel" "Next" here:

https://www.dummies.com/wp-content/uploads/293595.image1.jpg

Young users don't have such problems, but original iOS was easy to use for the older generations too. Steve Jobs cared about the easy discoverability. That is something that started as early as with Apple Lisa: https://www.youtube.com/watch?v=OW-atKrg0T4 also recently on HN: https://news.ycombinator.com/item?id=20247545

It was a big step back for my parents when iOS removed the buttons present in the older design and replaced them with the plain text.

You got distracted by the Rose Tinted Glasses Brigade, but I think you're absolutely right.

On top of that, I think there were other design considerations — how to make iOS' scale up to iPad more easily (remember when Siri was in a little box in the middle of the screen?) to the point where iPad apps can now be adapted with a checkbox to run on macOS. I think the groundwork started in iOS 7.