Despite slack being quite popular and truly a good product, Microsoft still has the upper hand. If you have the "Microsoft Suite" you get everything, access to all their products. Slack is fighting an uphill battle against Microsoft and eliminating Atlassian will help.

Dividing those who aren't diehard Microsoft fans won't help.

How is anything aside from legal infiltration going to make the playing field any less competitive? If we're all still playing by the same rules, there is still competition.