While yes, an engineer could build a slack clone. Probably initially quite poor and lacking features. If you're not in the business of building chat application

While yes, an engineer could build a slack clone. Probably initially quite poor and lacking features. If you're not in the business of building chat applications, having to actually maintain such an application becomes a burden. While you may save 500k for a few years. A few years down the road, when said engineer leaves, you will end up having to pay the cost either to exit said app, or spend a lot more engineering effort on it. There definitely is room for such applications, but a chat application probably won't set the business apart.