> You'd spend a lot of time and money on making one that would infect your mrsa strain, and mrsa would probably just start the arms race, leaving you back at the start.
Actually developing resistance against bacteriophages requires such invasive changes that most bacteria that develop it lose resistance against anti-biotics. That is why an anti-biotic therapy is often used together with a phage therapy.
> Actually developing resistance against bacteriophages requires such invasive changes that most bacteria that develop it lose resistance against anti-biotics
It requires some huge changes, sure. However, MRSA exists because Staph was able to adapt to be resistant to multiple antibiotics at the same time. There is no reason to assume that it could not adapt to become resistant to the phage used as well as some of the most used antibiotics.
Actually developing resistance against bacteriophages requires such invasive changes that most bacteria that develop it lose resistance against anti-biotics. That is why an anti-biotic therapy is often used together with a phage therapy.