Likeliness of this happening? I'm not a microbiologist, but given how we've been abusing antiobiotics for the past 60 years I honestly wouldn't be surprised if this became a reality in the next 20-30 years. In fact, WHO reckons we're on the brink, and we might not even get to see the effects of global warming take place because of it. Not to mention that global pandemics have practically caused as many deaths as WW2, with The Spanish Influenza taking the top spot at 75 million people. But the Spanish Influenza was just a small spurt of an epidemic. Smallpox on the other hand, is believed to have killed 300-500 million people over the course of the 20th century.

Obviously the likeliness of these diseases making a recurrence is pretty unlikely, but it must be noted that viral pathogens mutate to accommodate hosts all the time, and with bacterial infections like E-coli and Gonorrhea adapting to the effects of Antibiotics (there's only one type of Antibiotic left for Gonorrhea)  it would be dangerous to hand wave the idea of a deadly pathogen cropping up amidst an era where Antibiotics have essentially become futile .

So, could this relapse in Antibiotics in the near future be the most viable scenario for an extinction event to occur?

I can't wait until we get wiped out by an antibiotic resistant germ that will obliterate most, if not all, the human population