Theories of cosmological origins and ultimate nature of the Quantum will come and go, and be modified as new evidence becomes available. The rejection of some theories, modification and retry, and the proposal of new theories is way of science especially in Cosmology and Physics.
String theory remains a possible theory, and it is not rejected as a theory considering sciences own criteria. At present it is not favored by some scientists, though it does still have descriptive and predictive value in some basic properties of the basic particles of matter. String theory is a descriptive theory and is a theoretical framework describing particles as one dimensional strings, and it may be modified and tested in the future. Yes there are at present alternative theories and some have better descriptive and predictive value at present, none are of course conclusive, yet.
The following site does well explaining the theory in tracking the advances in string theory.
The Official String Theory Web Site
"A big complicating factor in understanding string cosmology is understanding string theories. String theories and M theory appear to be limiting cases of some bigger, more fundamental theory. Until that's sorted out, anything we think we know today is potentially up for grabs.
That being said, there are some basic issues in string theory cosmology:
1. Can string theory make any cosmological predictions relevant to Big Bang physics?
2. What happens to the extra dimensions?
3. Is there Inflation in string theory?
3. What can string theory tell us about quantum gravity and cosmology?"
Read further to better understand string theory.