While it may not be exactly what is asked for in the question "What strong opinion did you hold that has been changed over time?", here's my input:

At my first assignment as a US Air Force comm officer, we developed the system that managed cargo and passenger movement for Air Mobility Command. It was written in COBOL and had an in-garrison version that ran on a Sun/UNIX environment and a "remote" version that ran in a PC/DOC environment. Our data was stored in an "advanced file management system". There was an API that we used to read/write data and behind-the-scenes the data was stored in huge text files. About a year or so before I left, our system admins wanted us to convert the system to use a relational database. The development team fought it tooth-and-nail and won - no conversion of the system was ever done.

I often think "if only I knew then what I know now...". 😉