If the core developers do leave then they might form another company or disperse into other companies and continue to work on a fully-functional, Free toolkit. If one group or the other can't recruit a critical mass of developers then maybe all available versions bitrot or go on life support with a slow trajectory of features and fixes.
Maybe the worst case is if the development community splits into multiple proprietary forks, since with BSD license there is no enforcement of a software commons to collect contributions, then they probably all die over time, failing to achieve a critical mass of developers or customers, with features maybe folding in when individual forks go belly-up, or being lost.
Almost any scenario I can think of involves there being less developers working on any particular fork so as a consequence development must slow down (or stop), compared to a non-forked scenario, and upheaval for anyone who has a core dependency on the toolkit. It's probably a stronger dependency and harder to switch than MySQL/MariaDB to PostgreSQL for example.
If you are lucky then if there is a fork like this the bulk of the developers go to the Free software version, which then supplants the proprietary fork and becomes standard like LibreOffice vs. StarOffice, but you can't always rely on being lucky.
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