The evidence suggests that XML was never that popular though for the general audience, you have to admit.

For Web markup, as an industry we tried XHTML (HTML that was strictly XML) for a while, and that didn't stick, and now we have HTML5 which is much more lenient as it doesn't even require closing tags in some cases.

For data exchange, people vastly prefer JSON as an exchange format for its simplicity, or protobuf and friends for their efficiency.

As a configuration format, it has been vastly overtaken by YAML, TOML, and INI, due to their content-forward syntax.

Having said all this I know there are some popular tools that use XML like ClickHouse, Apple's launchd, ROS, etc. but these are relatively niche compared to (e.g.) HTML

MS Office and Open-/LibreOffice are using zipped xml files (e.g. .docx, .xlsx and .odt). Svg vector graphics is xml, the x in ajax stands for xml (although replaced by json by now). SOAP (probably counts as the predecessor of REST) is xml-based.

XML was definitely popular in the "well used" sense. How popular it was in the "well liked" sense can maybe be up for debate, but it was the best tool for the job at the time for alot of use cases.