…your description makes it sound like an upgrade, when actually the 256 configuration is just incompetent, a failure (or sabotage?) compared to the m1 models that worked properly without the expensive “upgrade”

    a failure (or sabotage?) compared to the m1 models that 
    worked properly without the expensive “upgrade”

I don't know the specifics, but the "lowest-capacity model has reduced performance because there's only a single chip instead of multiple chips being used in parallel" has been the case with SSDs for just about as long as they've been around and it is definitely not an Apple-specific thing.

My understanding is this: when the lowest-capacity model is only using a single chip, it's because the manufacturer is no longer making smaller sizes. For example, Apple's supplier(s) may not even be making 128GB NAND chips any more or may be discontinuing them soon, so they don't even have the option of using 128GB+128GB instead of 256GB.

I wouldn’t say “incompetent” if it still outperforms most x86 laptops you get for the same price.

Think of it as having one SSD fitted on the low end and two on all other models.