I worked at a company whose name begins with K ... exactly this happened. The 'old' UI was an ASP nightmare from the early 2000s but it contained all that the users loved. The new thing never took off, despite the CTO putting his job on the line for it.

Another company had a green-screen nightmare from the 1980s replaced by some mouse-driven mofre modern UI - ditto. The users could easily naviate with arrow keys, tabs, function keys. It was FAST. The new UI, and it's even newer HTML replacement, both needed the mouse.

Hard to sell and 2002 nightmare or a green-screen to a prospective client - but they LUURRRRVE a cool modern Angular UI, but the people who buy thing thing aren't the ones who have to use it.