As a reminder, SARS and MERS were contained thanks to massive international effort to identify the spreaders, quarantines (including whole hospitals), and restrictions on air travel.
For example: most new flus come out of China due to their agricultural practices; building a wall around China until they change the practices and isolating people in the rest of the world population who get it would actually remove the flu from the human experience. I believe flu killed 60,000 people in America last year. Are we ready to do that? Should only take a couple of years!
If a contagious disease with a 34% mortality rate don't justify containment. What does?
Your example of the flu is interesting. If humanity had modern resources to fight the ur-flu would it be worth the effort? How many lives would be saved over the generations if the flu hadn't become a common threat?
We're also dealing with people who completely forgot how hysterical people were over the swine flu, SARS, MERS and so on, which ultimately turned out to be nothingburgers. And a completely broken mass media system where pretty much all sources of news are about as reliable as the National Enquirer, and significantly more politically biased.
Flagging this was foolish and a further contribution to the mass hysteria.