And you're right – there are lots of challenges when migrating communities, both technical and sociological. I've done it a couple of times already. You can read about my process for one of them here. That was a couple of years ago now and Discourse has come along way. It also has good market share, so people are becoming more used to it (and it has features that we see in so many other popularly used platforms, like Facebook and Twitter).
I work as a community consultant, and part of that job is helping businesses to understand exactly what that direct value is. We get organisations to set ROI based objectives before building their community, and figuring out how they can measure success, and what success looks like. Examples of focussed business goals might be an increase in customers (advocacy, positive sentiment, sharing), reduced research costs (community highlighted problems & opportunities) or increased staff productivity (knowledge sharing). All of those things can have a direct effect on the bottom line, but take time to achieve.
I believe that migration of (at least some) content is important, but not everyone does. It is possible to flat archive old communities and start from scratch. That said, migration isn't that huge of a deal these days. Most new platforms have migration scripts written for all the older platforms.