surely all bikes are modular... they are all a selection of parts from various sources. It doesn't matter which part of the world those parts were assembled in.
What your looking for is a brand of bike, where the parts are available easily should you need them, and/or a brand of bike where the parts can be replaced or upgraded with others. That is one of the advantages that Bosch actually offers. Its a reliable unit itself, parts can be ordered from any Bosch dealer anywhere in Europe, and the rest of the bike is just a normal bike, so any bike shop even if they have no eBike experience can service and repair / replace parts if needed.
That's true, but some are more modular than others. On a typical Chinese bike,you can swap motors and controllers for different types and brands. The same with displays, throttles and sensors. Also, you can swap batteries for different types, sizes and brands. The bits are available from a large number of sources, and can typically be unplugged and swapped.The Bosch system has the sensors and controller integrated in the motor and there's no alternatives. There's also no alternatives to the battery and display, and even if yo can get replacements, you still need a dealer to set them up. I guess that the Panasonic and Yamaha systems are similar.
I'm thinking more about the guy with a broken bike that doesn't reside next door to a dealer he bought the bike from. Any person competent in basic electrics can fix a Chinese bike, but for a European one, you need special training, equipment and software, so if you live in the back of beyond, you're forked.
Think about the guy that wanted a new display for his Bosch bike. Even if he or his local bike shop could get one, it still wouldn't work without the special programming by a Bosch dealer, which they couldn't do without the bike (supposedly), so he had to send his bike a long way to get it fixed.