I'm not sure I understand the numbers here, and why $100/month seems to have been picked as a baseline number for a practice. If we assume the average practice gets around 500 patients, $100/month translates into $600k a year... even after malpractice insurance and whatever other fixed costs (record-keeping + office assistant), that's an EXTREMELY healthy and probably unsupportable amount of income for the average FM.
So even if the market becomes saturated... if we drop the price down to $40/month, with 500 patients that's till $200k a year gross, and probably $150k/year net... and with a much better lifestyle to boot.
It seems obvious to me that demand for this service is very elastic: drop down the cost enough, and there are many people that'd be interested.
To be honest though, I personally would be interested even at $100/month. My family's health insurance premiums are currently about $12k a year, paid by my employer. I also have two young kids at home, and it's inevitable that I stay up at least a few nights a year freaked out about their health... some nasty flu bug, or (imagined?) allergic reaction to peanuts/medicine... typical young parent stuff.
If I had the option, I absolutely would pay another $1k-$2k a year (or at least push my employer to offer it) for 24 hour, personalized coverage.