"There was a man in the land of Uz, whose name was Job; and that man was perfect and upright" (Job 1:1)
Hello, Houston, we have a problem...bleep.
As for your (and Calvinist favorite) verse, +Paul was reminding us that none of us is truly Christ-like but the importance is in how +Paul finished his sentence -- that everyone comes short of the glory of God.
All of us are born mortal and tainted with death. Compared to God we are always a "failure." But some among us are true saints and, within the context and constraints of humanity, some are capable of being righteous, even perfect, as the OT makes it plain.
"Hello, Houston, we have a problem...bleep."
No problem, Houston, Job said his righteousness was more than God's but when confronted with God Job repented,
Job 42:3 "Who [is] he that hideth counsel without knowledge? therefore have I uttered that I understood not; things too wonderful for me, which I knew not. Hear, I beseech thee, and I will speak: I will demand of thee, and declare thou unto me. I have heard of thee by the hearing of the ear: but now mine eye seeth thee. Wherefore I abhor [myself], and repent in dust and ashes."
Carry on Houston the interpretation was just an anomaly.
"But some among us are true saints and, within the context and constraints of humanity, some are capable of being righteous, even perfect, as the OT makes it plain."
What the Old Testament makes plainis that we have all sinned and none are righteous before God without the shedding of blood for sin.
Isa 64:6 "But we are all as an unclean [thing], and all our righteousnesses [are] as filthy rags; and we all do fade as a leaf; and our iniquities, like the wind, have taken us away. And [there is] none that calleth upon thy name, that stirreth up himself to take hold of thee: for thou hast hid thy face from us, and hast consumed us, because of our iniquities."
You use the phrase "within the context and constraints of humanity" however if man has true free will there should be no context and constraints. Every moral decision should stand on its own, but that, as you concede, is not the case. In fact every man makes morally incorrect (sin) decisions. If every one born, by necessity, came down with the same disease you would say it was inherited or congenital. Yet you deny the same concerning man's inclination to sin when faced with the overwhelming evidence. You say man is capable of being morally perfect and yet there has never been such a person, excepting our Lord, in all of history.