The short answer to "when is open sourcing a threat to business" is that it is a threat when it allows competitors an easier and cheaper path into the market.
If so, then why aren't open-core or dual-license easier and cheaper paths for competitors to enter the market? Even when the real money makers for a business are not in the core but in the non-free add-ons, isn't it still easier and cheaper for competitors to only build the add-ons than having to build both the core and the add-ons? Let's say A is the original company and B is the competitor. B can compete by:
- making its own add-ons, or
- packaging the core with its own add-on and release in a different brand
Why does A let B do that?