Sometime in the recent past year, Steam stopped working, and 'reinstalling' didn't work. So I finally investigated it today. I opened Console, launched the app, and then searched for 'steam' in the console log entries. I found this:
Unsupported 32-bit executable: "/Users/USERNAME/Library/Application Support/Steam/Steam.AppBundle/Steam/Contents/MacOS/steam_osx"
Ok... but I deleted the app and reinstalled??
So I pulled up terminal, and went to /Users/USERNAME/Library/Application Support/Steam/ and found that most of the files in this folder were dated 2018. :-/
Renamed /Users/USERNAME/Library/Application Support/Steam/ to /Users/USERNAME/Library/Application Support/Steam.old
Relaunch Steam and this time it started to download the latest update... OK good, now let's try redoing this with just renaming the Steam.AppBundle folder instead...
A couple of runs (update downloads, then update runs, then actually starting) and it worked.
