1. Right off the bat, there's the cost of building all of this + maintaining it. Something like Mixpanel at scale requires at least two, if not three, engineers: one for client libraries, one for infrastructure (which the OP blogs about), and another for the web app (dashboard, real-time user stream, etc.) To be sure, not everyone needs all features of Mixpanel. To be really sure, nobody needs all features of any SaaS tools. But if you really want to compare apples to apples, then you have to account for these. To hire competent software engineers who can collaborate closely and maintain a complex piece of analytics software requires at least $100k per year, if not twice that based on locale. That's at least $300k and more like $600k right there.
2. The whole point of something like Mixpanel is to make analytics accessible to non-engineers. In their case, it's primarily product people and secondarily marketing/customer success. In any case, building an analytics/data product consumable by non-technical people is hard and takes way more than assembling a couple of cloud infrastructure together. If there's one reason Mixpanel is still in business, that's because of this.
3. Finally, the OP has a valid point which should have been highlighted more, if not to make their own biases more clear: Mixpanel's diminishing differentiation is dev shops/consultancies' opportunity. It is indeed incredible that a dev shop can build even a third of Mixpanel's functionality by leveraging GCP components. Mixpanel had to build a lot of its core backend systems from the ground up, including its original key-value store. Just this year, they fully migrated to Google Cloud Platform themselves, suggesting that there's really little room for differentiation among analytics vendors at the level of infrastructure components (Mixpanel's arch-nemesis, Amplitude, leverages Apache Kafka and various AWS components, most notably Amazon Redshift, heavily)
With all of this being said, one thing remains true: the most expensive cost of any software is people running them and the dependencies created around them. These may not show up as line items, but they sure are deeply embedded in your total cost.