What will happen if I let strep throat go away by its self?

1st Sx will . Strep throat goes through several phases. The acute symptoms will eventually fade in most without rx. During the sub-acute phase, the body continues to make antibodies and a fraction (1/200) develop long term complications including kidney damage, heart damage(rheumatic fever) and brain injury.Antibiotics can stop the complications if started within ~10 days of first fever.
Strep throat. This is always risky, not because of the infection itself, but rather because of post-streptococcal complications like acute rheumatic fever.