>Because almost certainly engineers will want to integrate an on-premises system into their existing system, and that will trigger the AGPL for all the systems that are network-connected to the AGPL system.
That's the FUD version.
The real version is that unless you are changing the source code (hint: not just writing a plugin using an externally facing API) and deploying it, you don't have to do shit.
And you still don't have to do shit even if you are changing the source code, as long as you only deploy it internally, which doesn't count as redistribution.
The key thing is MongoDB uses an AGPL _variant_ that has AGPL core and Apache 2.0 drivers. So you can run it in proprietary software if you're only calling the Apache 2.0 drivers and not modifying the AGPL part (MySQL is the grand-daddy of this variant model back to the days of GPLv2).
A number of other projects use the same pattern, and so does Mattermost. If you can run MongoDB there shouldn't be a licensing issue with Mattermost.
That's the FUD version.
The real version is that unless you are changing the source code (hint: not just writing a plugin using an externally facing API) and deploying it, you don't have to do shit.