It’s a real threat, but not an insurmountable one.
Yes, we probably can make new antibiotics, via a wide variety of methods, including plenty that we haven’t thought of. But there’s no guarantee that we’ll be able to. At some point, we may hit some real barriers. In the 60s, everyone thought that we’d all have our own private jet planes by now, but it turns out that there were physical and economic barriers to that sort of progress, so we don’t. It’s hard to predict what kinds of barriers we’ll run into that will slow or halt development of new antibiotics.
The additional wrinkle is that many of the problems with antibiotic use and resistance are cultural, economic, and systemic, rather than medical in nature. These are classic tragedy of the commons issues. Antibiotics are overused because each individual actor has an incentive to use more, even though the net effects of everyone using more are worse. Farmers have an incentive to use antibiotics on their animals because they result in higher weight animals and more money. And if they don’t use them, their animals might all get sick (and some competitor who did use them will drive them out of business). Doctors and patients have incentive to take them because, even if they’re pretty sure that it’s something viral, a small chance of dying right now isn’t worth hypothetical future use. Pharmaceutical companies have more financial incentives to invest in things other than antibiotics both because (1) new antibiotics are generally held back as much as possible to avoid resistance to them spreading, which means that they don’t get paid as much and (2) there’s lots of social and governmental pressure to price actual life-saving treatments closer to marginal costs. No tearfully distraught woman is going to get a bunch of TV time cradling her poor husband’s pale head because they couldn’t afford a new baldness cure; the mother of a kid who died because the antibiotics were too dear will play very well.
Some of these things can be fixed or helped by better regulation and market making (prizes instead of patents for new antibiotics, perhaps). But (maybe you’ve noticed), our governments aren’t very good at working with each other (or with themselves). And our public health resources have their attention split between new treatments for illnesses and convincing people to actually use preventative treatments we figured out were effective 150 years ago.
The bright side is that we have disciplines like genetics and computing that will potentially allow us to leap so far ahead of evolved resistance that we can waltz circles around it. The speed of evolutionary adaptation in microbes is fast (they reproduce really quickly), but relatively fixed. The speed of our computers and DNA sequencers and protein folding models keeps increasing. If we can actually maintain that increase, we should get to the point where new drug discovery can outpace evolution, and then say goodbye to microbial diseases for good.