MS Teams is an Electron app too. This very post is about React Native for Windows and Mac, which are being developed by MS because they're used for the front-ends of much of the rest of Office 365. Microsoft clearly thinks that it's worth rewriting large parts of their crown jewels using cross-platform frameworks.
Hmm this one is tough Office 365 has more paying users on teams. I doubt anyone is paying for teams, also they are not direct competitors. We use both, likely to get phones with teams in the future as well.
Whether it is fair or not that Teams comes with Office is irrelevant. This is Slack's competition. They have to convince companies that it is worth paying extra for their product.
In the famous words of Jobs about Dropbox -- chat isn't a product, it's a feature.
How many people know that Slack is battery draining monstrosity?
The only people that pay for Slack are companies. The definition of Enterprise Software is where the user is not the customer.