Transplant surgery becomes virtually impossible. Organ recipients have to take immune-suppressing drugs for life to stop rejection of a new heart or kidney. Their immune systems cannot fight off life-threatening infections without antibiotics.
Removing a burst appendix becomes a dangerous operation once again. Patients are routinely given antibiotics after surgery to prevent the wound becoming infected by bacteria. If bacteria get into the bloodstream, they can cause life-threatening septicaemia.At least then we don't need to worry about dying of cancer, because, chances are, pneumonia, TB or cholera (or, indeed, complications from childbirth or sporting accidents or other things people don't worry about these days) will have picked most of us off before then.
Of course, unlike in the pre-antibiotic era, we have technological advancements such as bioinformatics, which opens the possibility of new treatments being developed. However, no such deus ex machina appears on the horizon yet, and the clock is ticking.
The irony is is that Cholera, Pneumonia and TB, along with Malaria are already killing millions of people today, they just happen to be mostly black and always poor and therefore don't seem to count, even in the Guardian. You only ever rarely use antibiotics to treat Cholera, it's self-limiting and unlikely, thanks to piped water, to ever make a resurgence in the west. Makes for nice copy though and being a disease I'm sure 99.9% of readers have never come into contact with it sure sounds a lot scarier than it actually is.