The plants which are growing at the moment have been produced by seeds of last years flowers. They do not 'run out of gas' but over several generations, will often change/revert to becoming closer to the original species...which, in annual poppies (papaver somniferum) do tend towards single blooms of a pale grey/pink with darker central blotches. Size may also be affected (but more likely, it will be variables such as weather, soil fertility, amount of sunlight/water, competition and so on.

As for reseeding, it is probably more likely that the cosmos did reseed, back last year, but the seeds have failed for any number of reasons.