The summons is extended, but concentration is no longer the duration of the spell any longer. Concentration is the actual duration of the spell, that is what is extended by Planar binding. It works exactly like a higher level casting of bestow curse where the duration changes from concentration to a fixed amount of time.
It is also worth noting you can not send the demon (or other summons) back by ending concentration once you cast planar binding and there is no option to end the planar binding early. Once you cast it the creature is bound until the planar binding ends.
There is also a logical falacy to this interpretation - If planar binding lasts 24 hours ..... and the spell is extended to the duration of planar binding (24 hours), and then if the spell ends before 24 hours because you lost concentration ..... then it was in fact it was not extended to the duration of the planar binding.
Finally it is impossible to concentrate while unconscious, so if this were true you could not even use planar binding for 180 days like it says you can on a 8th level casting, because you would need to sleep and could not remain conscious that entire time.