It depends. The MySQL -> MariaDB was an outcome resulting from several
factors. Among them, Oracle's open hostility to the larger free and
open source software community, the departure of Monty from MySQL to
fork MariaDB, etc. Additionally, it was not an overnight change. In
fact, MySQL continues in Debian, just without security team support.
There are similarities there with what happened with OpenOffice.org ->
LibreOffice and XFree86 -> X.org.
So, if F5 is as skilled as Oracle in being hostile to the communities
and the original founder(s) throw support in another direction, then it
may very well be that nginx is replaced by a fork. The more likely
outcome, though, is that the fork will become preferred by the original
However, it will not happen "just because" and will take quite some time
to fully manifest itself.