Besides Teams continuing to grow at a rapid rate (largely due to the popularity of Office 365), we can expect all of the competitors to continue to introduce new functionality to remain competitive. For example, Google will evolve their Hangouts platform, having recently split the app into Chat and Meet platforms, which will focus on more Slack-like collaborative team chat and videoconferencing, respectively. Slack will have to continue to be innovative because most businesses use either Microsoft’s or Google’s productivity suites, which in many cases come bundled with their business chat applications at no additional cost. So in many ways, business chat app adoption will be a reflection of — or perhaps an extension of — the popularity of productivity suites.
Slack has already turned down many offers in the past, and Microsoft now has their own Slack competitor, an acquisition is not necessarily in the cards. Besides, it was rumored that Microsoft considered making an offer for Slack a few years ago, but ultimately decided not to.