Having been involved in designing and building ultrasound systems for a couple of decades, I thought I'd comment on this in a blog post for those looking for a little more detail on the issues involved in building and selling ultrasound devices for medical use.
A quick summary - they're not expensive, in fact for what you get they are remarkably inexpensive. There's a huge amount or work that goes into them, and the author of the original piece simply doesn't even know enough about the subject to realise he doesn't know what he's talking about.
Unfortunately, there is only a single line in the linked blog post regarding the economics of the situation : "made without the benefit of mass volumes (no millions of devices here)". The rest of the post is essentially meaningless, since it doesn't address the cost issue. The complexity in a sub-1k$ mobile phone is far greater than in a typical ultrasound imaging device that is currently in the field, but the economics of mass production enable massive reductions in price.
In other words, you've provided some suggestions regarding the difficulty of the task, but have not in any way proven or convincingly demonstrated that sub-1k$ ultrasound imaging devices are an impossibility.
From my POV, having also spent many years designing imaging systems and other devices to measure the human body (ultrasound, EEG, EKG, MRI), the biggest challenge to providing cheap consumer-grade tech here is the regulatory burden. If anyone wants to buy a sub-1k$ ultrasound imaging device, there are plenty of unapproved models on Alibaba that work just fine -- provided you're willing to test the device yourself for safety, to ensure it meets your own risk tolerances.
http://liesandstartuppr.blogspot.com/2016/12/why-are-medical...
A quick summary - they're not expensive, in fact for what you get they are remarkably inexpensive. There's a huge amount or work that goes into them, and the author of the original piece simply doesn't even know enough about the subject to realise he doesn't know what he's talking about.