there's an ongoing thing with increasing automation reducing the number of human beings required to operate an X. Trains and metro transit systems are an early example - with similar safety arguments being made. If one pilot is enough, you're kind of making the argument that no pilot is enough, with one ready to step in in case of emergency. In the same way that currently we need one pilot, which is why we have one pilot plus a backup in case that pilot becomes unavailable.
Hence the argument from the pilots side that "one means none". I agree, but I'm also impressed enough with automation in aviation that we should take it seriously and get some empirical numbers on whether "none" is enough.
Would you board a pilotless flight?