And how robust can it be to the opinion that twentieth-century Jews were killed because of a curse of the old covenant?
It's VERY robust, because it's laid out and stated in the Old Testament that if they fail to obey God in any egregious fashion which involves the outright rejection of God's Will, then God has stated that He will ESCALATE various levels of Curse(s) [plural] against His people.
I mean, even we Christians, if we don't behave, may be subject to God death penalty just for perpetuating sin in this life. How much more so will God punish those who put Him to the test, even by those who claim His name and yet reject His Son in an ongoing manner, century by century by century?
Again, you're going to have to get familiar with the Old Testament, particularly with the Law and it's pronouncement of Blessings and Curses that are therein delineated ... twice in fact. So, yes, my position has Scripture and Eschatology to back it up rather than simply leaning on our own weak intuitions about ethics and morality, as people often do today through using casually strewn together moral notions and acting as if there is actually some kind of Unquestionable Substance to it all, as if their views are somehow a kind of sacred cow that one can't touch.
No, if we have any veneer of humanity and sense of human dignity and significance, it's because it has been surreptitiously lifted from the Bible, modified, and then reused without giving God and Christ credit for their having injecting new notions of human dignity into the core of human ethics. Yet, we pat ourselves on the back for what we think is our own moral ingenuity when that supposed fact is a half-truth.
So, yeah. I'm sorry to have to argue this point with you because I know you're a good hearted guy, but there is a reason there are numerous warnings to humanity and to the people of God littered throughout the Bible. Some think God was just kidding; the fact is,
He wasn't, and we see this again numerous times in the Old Testament not the least of which include the time when Nebuchadnezzar wiped out most of Jerusalem and the 1st Temple, killing much of the Jewish population and hauling a small remainder to Babylon, and then also several hundred years later when Vespasian and Titus lead the Roman Legions against the same place and wiped out and/or displaced the Jewish people of Jerusalem and the 2nd Temple. And now, for the last two millennia, the non-christian Jews have unfortunately been homeless and subject to various pogroms and oppressive measures by Europeans and Muslim peoples.
We see the pattern not only in the accounts of the bible, but then again repeated in the last 2,000 years of history, with 1948 being another little point that fits in with what God has stated in the O.T.
So, there is
ALL of this to consider; but unfortunately many people just don't .... want to look. Well, it is what it is and it has been what it has been. It is sad, but it is true, and my heart for the non-Christian Jewish people, wherever they may be, is that they'll turn to Jesus of Nazareth who IS their Messiah even though they have tended to leave Him unrecognized.