Calling Buddhism or Confucianism not a religion is a political claim which the speakers usually have zero knowledge of and I’m sick of it. The history of what is religion in China from the republic onwards with the very Protestant understanding of what is a world religion is very important to understand this question. But most people either claim this for their hippy means or to be derogatory
I mean, it really comes down to how you define religion. A lot of people's conception of religion is grounded in the Abrahamic traditions (Christianity, Judaism, and Islam), and other traditions don't fit neatly into that framework. It's useful to identify the Christian/Abrahamic lens through which various traditions have been historically seen and to reexamine whether the actual reality is in line with the picture we have of them. Whether such traditions are ultimately classified as religions or not is semantic, but it's worthwhile to examine them as they are and to question assumptions about them.
You’re 100% right. Religious studies went through exactly what you described. Thinking of religion as Abrahamic, opening up to world religions through the former lens, opening up to more that would have been considered cults or bad, then now I think quite open to most religions. Defining religion is a boring but important part of the field that many monographs feel it’s their duty to do.