Why are red blood cells considered to be cells?
A very good question, and it is most likely because of the last option.  It had a nucleus for part of its life. After the RBC jettisons its nucleus, it still remains very metabolically active for approximately 3 months.  It maintains its cell membrane integrity, it metabolizes glucose, it interacts constantly with its environment, numerous cellular functions and structure remain intact...  It is extremely specialized for a primary purpose, and no longer requires the nucleus to provide more proteins.  It has limited capacity to heal from injury, so it has a limited life span.

Speculation:  I wonder if it might lose the nucleus early on so that when it is destroyed in the spleen at the end of its life as RBCs are, the spleen macrophages are not overwhelmed with additional processing of nucleic acids?  Macrophage type cells are already working hard in there to clear infectious agents and some immune cells from the blood.