The reason we need to use a license like that and not a libre license is that some of our software is very niche and would only be useful to a handful of companies around the world.

It's quite likely that none of these other companies employ any programmers at all bar the occasional contractor. Basically we are trying to disrupt an industry (in a small way) by getting more efficiency by doing stuff through tech that they are doing by employing legions of admin staff and using the old-boys network (it's a very old school industry).

If the source for certain parts of our software was available to them then they would most likely simply use it (employing an Indian contractor or similar for a couple of weeks to set it up) to wipe out our advantage and not contribute much if anything back to us.

There are however other parts of the software that could be generally useful to a wide number of people who are much more likely to contribute code back.