In effect you are right, the signaller will know when a train has left
the affected section and be able to authorise a train to pass the
affected signal BUT not over the train radio, BUT when passing a signal
failure red the train will then be stuck doing "a speed at which you can
stop short of any obstruction" in other words the train will be doing
under 10 mph (if you try to exceed 10mph the brakes will come on), 5mph
or less is more likely (There's an alert noise if you go over 7mph,
doing 5mph keeps the cab noise down!) this is for 2 correctly working
signals, if the next one is also failing (not uncommon) then the whole
process has to be started again.

Sounds like the system needs to revised then, or perhaps put some
backup sensors in the system or perhaps the block the signal controlled
be taken over by another signal so you have a double sized block
allowing
trains to pass and allowing the duff signal to be taken out of service
and
repaired. Its not beyond the wit of man to come up with a solution that
doesn't involve incoveniencing thousands of people when you've had over
a century to think one up.

B2003