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!
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?
If a contagious disease with a 34% mortality rate don't justify containment. What does?