Now, i will say, this is at least an argument that is respectable, as it is based around facts and measurable ideas..
That said, while i agree with everything you say about interference, i feel like i must point out that DAPs like the X7, Onkyo X1, run into the same basic problems. They still face the radio interference (2.4/5Ghz) and sensor interference inherent in the phones, though on a slightly reduced scale with no cellular radio frequencies.
Your third bullet in a way is what i feel is the big recent shift in devices. New phones like the ones we are discussing have the dedicated DAC/AMP hardware that other dedicated DAPs have (yes, in addition to the native ADC/DAC), which are optimized for music. Now, i will grant that the DAP software may be in some way further optimized, but without looking at source, its hard to know what is changed for certain.
But you are fully correct, that Cell phones do have slightly higher space restrictions than DAPs, though by no means are the DAPs much larger. The pictures of the Onkyo X1 board shows that the DAC/AMP board is fairly sparse; i am curious how the sound would be impacted had it been all placed on the same board. Having looked at the V10 teardown, i can note that the DAC/AMP setup there is on the opposite end of the phone from the CPU and memory, though it is all one physical board.
As it is, i see the best of both worlds in pushing for phones like this. With my purchasing the V10, i was able to leave behind my X5/E5 combo, and gained the convinience of a MUCH MUCH better UI, replaceable battery, and more features. I acknowledge there may be an SQ hit compared to something like the Onkyo, but am willing to take what there is for the portability of an all-in-one.
