Why isn’t the modern buckle only for 40m Apple Watch?

If you want a modern buckle, then you will need to reach outside of Apple and go to Amazon. Look for SUPCASE Rugged Protective Case. I have one for the 44mm. There's no way I want this watch falling off because of a failed magnetic latch.

They should, since the 38mm bands will fit the 40mm watch. I would say they are interchangeable. Worse case scenario is that you return it for a refund if it doesn't.

No one here could answer that question. Only Apple would be able to and they do not participate in the forum. However, the 40mm bands will fit the 38mm watch. They are interchangeable.

the 40 mm bands are interchangable with the 38 mm watch as are the 44 mm bands fitting the 42 mm watches.