It doesn't seem aligned to you having read the body of your text, but something I had a knee-jerk reaction to as an answer to 'future-proof' is Mainframe, COBOL

It doesn't seem aligned to you having read the body of your text, but something I had a knee-jerk reaction to as an answer to 'future-proof' is Mainframe, COBOL, DB2. Yes, even today. Backed by a strong business understanding, these are great skills.