Most (but not all) viruses end up killing the cells they infect, so for those a fusion inhibitor should work fine. Not sure where RSV fits on the spectrum, but it does look like it makes infected cells very sensitive to apoptosis by TRAIL: Respiratory Syncytial Virus Infection Sensitizes Cells to Apoptosis Mediated by Tumor Necrosis Factor-Related Apoptosis-Inducing Ligand https://www.ncbi.nlm.nih.gov/pmc/articles/PMC187410/