Viruses and bacteria evolve over time and can develop a resistance to antibiotics. In theory, this resistance can also apply to phages, but it may be less difficult to overcome compared to antibiotics.
Because phages are target specific, meaning only a one or very few bacterial strains are targeted upon, it is easier to develop new phages than new antibiotics. A time period of only a few days or weeks is needed to acquire new phages for resistant strains of bacteria, whereas it can take years to obtain new antibiotics. When resisting bacteria evolve, the assigned phages also evolve, so when super bacterium appears, an equivalent super phage fights it as long as the phage is derived from the same environment.