I obviously don't agree to that, it has to be economically viable to exist. Are you suggesting a big con?
My retort based on on-the-ground experience your armchair lacks:
Since you appear to be referring to the smartphone device which does not replace consumed heat, then the assumption is the users are too dumb to count variability in their electricity bill. The designers will presumably keep the targeted electricity below the normal variability on the typical electric bill (or below the differences that consumers pay attention to), larger battery, larger charger, and thus the user may not notice.
Remember the target market are dumb (ignorant), non-technical users in the developing world.
Any one who claims this target market is going to care that their device has been captured for mining without their active knowledge and participation, has not spent enough time
with ladies in the Philippines and their interaction with their mobile phones.
I have relatives in the Philippines who live in a squatter area. I know exactly how these people manage their lives and electric bill they don't.
In short, they have no discipline whatsoever and run up huge electric bills then pay it off with short-term debts. They are perpetually in debt.
Many of you all apparently have some incorrect fantasies about the developing world. It would help if you actually had experience.