They understood the GUI couldn’t be everything but NIH was definitely in full swing at that time. It was enforced from the top: If you wanted to use an external product and MSFT had a competitor you needed VP approval. Same went for Open Source. Only the most senior people would even attempt it, and it was understood they were spending political capital.

The one place this didn’t apply was your choice of code editor. You could use whatever you wanted and that had long been tradition. I spent a lot of effort getting VIM to work only to realize it didn’t scale well to multi-million LOC code bases.

As someone who uses Vim on relatively large files daily (500k - 1.5M lines), where were the hangups?

The speed of navigating the code base. Ctags existed but people using SlickEdit or Source Insight were able to navigate the code base dramatically faster. As a new hire I wasn't going to get code annotations emitted as part of the build, so an editor with a proper parser was key.

Really had nothing to do with the size of the file.

To be fair to Kodak, it is very hard to invent digital photography but fail greatly to use it. Or Xerox, ... or IBM who is the largest PC seller exactly the time when it was overwhelmed by Microsoft.

An organisation large enough would have middle management and people who promote from within. And they are successful in one culture and it worked even for customers.

Then ...