Indeed, if you know how to design a custom PCB, have it made, and write embedded software, then Jacdac might be an overkill for you.

The Qwiic cables have to be really short (Jacdac cables can be a few meters) and are not designed for easy plug-in and out thousands of times. Also, the I2C devices are not standardized meaning you need a new driver in your software environment for every single accelerometer. Finally, I2C doesn't let you have multiple instances of the same device because of address collisions.