Based on my experience working at an analytics company and running one myself, what this post misses out is that an increasing number of people working with data today are not engineers. These people can range from product managers who are trying to figure out what features the company should focus on building, marketers to figure out how to drive more traffic to their website, or even the CEO trying to understand how their business as a whole is doing.
For that reason, you'll still see many companies pay for full stack analytics tools (Mixpanel, Amplitude, Heap) in addition to building out their own data stack internally. It's becoming more and more important that the data is accessible to everyone at your company including the non-technical users. If you try to get everyone to use your own in-house built system, that's not going to happen.