It's quite possible. I'm using MOBILE tokens earned from what Helium calls "Discovery Mapping" to pay for Helium service. It is admittedly easier for me to cover the $5 cost of my plan (I got in when South Florida was Helium's test market) than the current $20 cost.
For what it's worth, the recent trend is I'm earning more tokens. I was averaging 400 a day but recently it's been 450. Generally, your analysis is correct but I guess, for now, those participating in mapping is down.
The ctiteria is a minimum of 40 hexes per day, which indeed works out to around a mile.
I suppose it depends on what one does besides work on a daily basis. I run 3 miles most days, so, for me, covering the distance is easy. It could just as easily be covered walking, biking, or driving while running errands. At least, where I live, it's not difficult covering a mile when out and about for whatever reason.
I would have thought so as well but can't say I've noticed significant battery drain. I also tend to toggle on Discovery Mapping in Helium's app, get my run (or other distance covering activity) in then toggle Discovery Mapping off otherwise.
I currently have 25,625 MOBILE tokens in my Helium wallet. The value of the tokens like all currencies (crypto or otherwise) fluctuates but today that translates into around $35. The tokens don't expire.
I'll go ahead and add my Helium referral code to the appropriate topic in case there's interest. The referral gets you (and me) $5 toward monthly service.