Earlier quoted context omitted.
And then the user goes out for the day, opens up the app, and wonders why the last 3 days of data is missing. Meanwhile the chip that does Bluetooth also just has wifi bundled in. Aside from the security risk, directly connecting to wifi is a vastly superior experience.
How much data can a toothbrush collect? Surely just a few hundred bytes per brushing session. The ESP32 has 160 kB of usable RAM out of the 520 kB total capacity. Surely enough for weeks of data even if the data structures are badly designed.
The user expects the data on the app to always be in sync. If they check it outside of bluetooth range and see days of data is missing, it'll look broken.