Most mutations probably just render the thing unable to replicate and it "dies," much the same way flipping a random bit is more likely to cause a program crash

Most mutations probably just render the thing unable to replicate and it "dies," much the same way flipping a random bit is more likely to cause a program crash than do anything useful at all. The relative rate doesn't matter, the absolute rate does (how often does it mutate at all and survive to infect another host).