My own CPR instructor made it plain to us, his students, that CPR is only an emergency stopgap measure to be employed while awaiting EMT assistance.
Anyone who is in cardiac arrest is, to all intents, a dead person already and even poorly administered CPR cannot possibly do any more damage than has already taken place. So it is worth trying in any case.
However, doing CPR is exhausting. Even running a team of people at the victim can only buy you so much time while waiting. If assistance is too remote, the victim dies.
Coming away from that class, I realized that, although I might suvive a mild to moderate cardiac infarction in the backcountry, if I suffered a cardiac arrest I would not survive under any imaginable circumstances.