The topic of leaving decay under a restoration has been researched over several decades; even without using SDF as a liner.  Edwina Kidd did much of this research and it can be seen in PubMed just by searching her name.  She showed that bacteria may exist, but the decay doesn’t progress when cut off from the oral environment.  The hall crown technique also proved this.  SDF as a liner has proved this again.  More recently Rella Christensen has shown that bacteria exist under SDF treated sites and under restorations, but the decay doesn’t progress.
In short, you will never get a sterile environment, even if you remove all of the decay.  But, the lesion will stop in most cases.  Adding SDF is the belt and suspenders method to ensure this takes place.
Back in the Amalgam days, Silver nitrate was used in this manner which is why those amalgam restorations lasted decades.