People who pay are your customers. Everyone else is free advertising / marketing / product or service testing / feedback group.
Note that "who pay" doesn't necessarily include "those who might pay" but can include "those who would pay".
The ones wanting your site open source more likely fall into the "might pay" group. I've used a plugin architecture to test the resolve of "might pay" vs "would pay" groups. By exposing APIs for at least import/export I removed the downside of "data held hostage". "Might pay" also frequently turns out to be "won't pay" most of the time.
I wouldn't recommend you open source your site unless you see a lot of potential for open source contribution or where review[1] is expected. I would suggest considering some sort of plugin or API for users. This is a good enough bridge for most people. Especially business to business linkages or even b2c.
[1] crypto is an example where review is worthwhile.