My question is here : Please convince me I have to
make my propriety
software open source.
You don't have to make it open source. I won't try to convince you
that you need to. I will, on the other hand, try to convince you that
you should open source it.
1. If your extensions are really nice, the community will use them,
and will likely improve them for you. I've found this to be the case
with the LDAP extension.
2. The community will find bugs in your software that your company
hasn't noticed. This is especially useful for security.
3. It will garner respect and good will for your company in the community.
4. If you are not selling the software to others, it can honestly only
benefit your company; internal software development is expensive, and
this is a possibility to share that development cost with others
inside and outside of your industry. Even if you are selling the
software, you can switch to a support model. See Red Hat, and Sun
Microsystems as examples (ignoring the fact that Sun is dying due to
the Oracle merger).
5. These extensions may strengthen the MediaWiki enterprise community,
which isn't a goal of the Wikimedia Foundation. The enterprise
community needs some champions, and you can be one of them. Doing so
makes it more likely that MediaWiki will be used in the enterprise,
and will result in more enterprise features and extensions that will
be useful for your company.
I look forward to having another enterprise member adding their
contributions as open source. If you do release your extensions,
please ask for commit access, and add them to MediaWiki's SVN.