I fully agree with the premise and details of your post; to the last point of "can you name a proprietary...", HN tends to forget about / disregard the wonderful world of "enterprise" / "big corporate" software. Most large companies I dealt with are very "modern"/open-source on the client-visible apps; but very proprietary / legacy on the back-office stuff, from ERP (Enterprise Resource Planning - HR/Financials/CRM/EPM and such) onwards. I mean, corporate world back-office still largely runs on DB2 and Oracle databases, the likes of Windows & AIX (HP-UX/Solaris back in the day as well), even z/OS, and it goes up & down the stack like that.

(as a somewhat related stunning bottoms-up anecdote, a young-ish AIX sysadmin at my recent engagement absolutely positively refused to install AIX tools [1] on the servers, as he didn't want to pollute them with that "unsupported open source stuff")

[1] https://www.ibm.com/developerworks/aix/library/aix-toolbox/i...