It sounds like someone who only had bad relationships and then proceeds to say that all men/women are trash and go M/WGTWO. The world is very big and each one of us can only experience first-hand a tiny part of it.

As other suggested, to understand the roots of Agile, I suggested looking at the Manifesto itself. Additionally, I suggest looking what people like Bob Martin, Martin Fowley, Allen Houb, Kent Beck, and Dave Farley are saying.

E.g. https://www.youtube.com/watch?v=hxXmTnb3mFU https://www.youtube.com/watch?v=4JihsBOBbdI

For reports on teams' performance, I suggest looking at the DORA's report. There thousand of teams have shown they experience in the last ten years. Additionally, if you want to some academic researches, going to Google Scholar and searching for "Systematic Review PUT_HERE_ONE_AGILE_PRACTICE" generally yields good results.

https://www.sciencedirect.com/science/article/pii/S095058490... https://ieeexplore.ieee.org/abstract/document/5654783/