$500 a month for US coverage might be "reasonable", but not always affordable. A lot of people have to retire on less than 20,000 a year. Also, my understanding is that TIS rates are for Canadians who maintain their provincial coverage - not for somebody living in the US more than 7 months a year. Also, TIS plans require a questionnaire, same as most plans for people over 60-65. Make one mistake and you are finished.
If you only need Mex insurance, it's relatively cheap even after the age of 70-75. TIS apparently is offering a US coverage, or "any country" (because when it covers US, any other country is cheaper yet). Doesn't look like they have something for "Mexico only".
Yes, some snowbirds in Mex buy a Mex coverage, and some are keeping just Evac + provincial plan. They cover minor ailments and diagnostics out of pocket and then get reimbursed a few hundred by the province, no big deal even if province won't pay.
But keeping either Mex insurance OR evac+provincial may not be enough if something serious happens. If you only have a provincial - good hospitals in Mexico are private, the cost may exceed the provincial coverage a little. Or you have sufficient Mex coverage but Mexican hospital can't help you and brings you to the closest US hospital because you can't be taken to Canada by air - then without US coverage you have a problem. Even if your medical condition allows to take you to Canada, very few Evac plans will take you to the hospital of your choice, vast majority of plans will bring you to the "closest capable" hospital, which is of course US.
Medjet is one of Evac plans that offer a choice of hospital.