Opening Pre-2018 InDesign files

I can't believe I'm the only user who needs to access pre-2018 files, and did not have the foresight to go back while I did have access and re-save all the older files with the .idml extension.

Now, if I need to open a .indd file from, say, 2015, the latest and greatest version of the program throws me an error message that "Cannot open the file "xxxxx.indd" Adobe InDesign may not support the file format, a plug-in that supports the file format may be missing, or the file may be open in another application."

No luck.

I did try one trick. I changed the extension manually in the Mac OS finder window of an archived 2015 file, from .indd to .idml and I was able to open that version.

So is this a workaround for this problem? Seems very duct-tapey and paper-clippy for a problem involving older files. But, who knows.

Any thoughts?