The bacteria has had far more than 30 years to develop resistance, and it could do it in stages. We don't know how many other bacteria over the gigayears managed to poison itself.
True, but for one, the antibiotic will be produced and used in really large doses once it enters the market, meaning there are trillions of bacterial cells encountering it, which raises the likelihood of development of resistance. And second, horizontal gene transfer between bacteria means different strains of bacteria can cooperate in defeating it (https://en.wikipedia.org/wiki/Horizontal_gene_transfer)
