Reading your reply I wondered if I, and other Jews, would feel less worried about intermarriage if there were a billion Jews in the world. Perhaps a dialogue on intermarriage between a Jew and a Chinese-American is impossible, given the enormous numeric disproportion. But let's try.
You conclude by asking whether "In America, mustn't any ethnic identity adapt to survive?" Right you are--but being a Jew isn't an ethnic identity. Or at least, it isn't if you take it at all seriously. For those Jewish friends of yours (to whom being Jews means having "intermittent access to a heritage that has dissipated from disuse"), it's probably an identity much like that of Irish- or Italian-Americans. Their "Jewishness" consists of recipes, maybe some holidays, and memories of immigrant grandparents. It has indeed dissipated as the old folks die, as your friends forget about the ethnic customs, and as they move away physically from the historic centers of Jewish ethnic life and to the new cities of the Sunbelt. You and I would agree entirely that such an identity must "adapt," and moreover that such an identity will never be and should never be a barrier to intermarriage. To refuse to marry out of the Jewish community for such reasons is silly, if not, in fact, tinged with racism.
But there's an important word missing from your letter to me: God. For any serious Jew, "being Jewish" isn't an ethnic or "ethnocultural" identity at all, it's a religion. And it couldn't be clearer what the religion requires: In one of the central prayers of Judaism, the Sh'ma, we repeat, every day, God's commandment that: "These words which I command thee this day shall be in thy heart. ... And thou shalt teach them diligently unto your children." That is the Jewish view, whether some Jews take issue with it or not. Reading your words about not being able to imagine requiring your own kids to "adopt any single way," I figure that you are not committed to any faith--Jewish, Protestant, Catholic, Muslim, or any other--or I doubt you would think that way. It's a free country, and you actually can't "require" it, but you sure can teach it to them; and if yours is a living faith you would try very hard to give them an understanding of what it means to you. If that faith brought with it the weight of Jewish history, you would, I think, also want to weep if you failed. That this is a free country means we may not coerce fidelity; that this is an open society means the risks of assimilation are high; but that we are Jews means we must raise our children within our religion and hope and pray they will choose to stay within it.
Many Jews are choosing to disregard their religion and fade gracefully into the ethnic patchwork, which is part of America's glory. That is, as you correctly note, one of the costs and one of the benefits of living in America. But let's be clear: Intermarriage makes it very unlikely that a Jew's children, and even less likely that his or her grandchildren, will be Jews. If Judaism the religion means nothing to us, then I agree: Why object? But if it does, if we believe in God and practice Judaism, then we must object.
You are right to ask why the continuity of "Jewish-American identity" is such a big deal. It isn't, any more than Jewish-Argentine identity is. What matters, to the believer, is the continuity of Judaism. How does one translate this into a meaningful message to other Jews? If they have already pretty well drifted away and lost their faith, it may be too late. My main concern is to be sure that those who really want their children and grandchildren to be Jews recognize what forms of behavior make this likely and what forms make it unlikely. (For example, taking your children to synagogue or getting them a good Jewish education helps; marrying a non-Jew hurts.)
To return to my initial remark, I'm sure I would be more relaxed about all this if there were a billion Jews in the world and if our numbers had not so recently been vastly reduced. But it isn't so, and the Jewish homeland in Israel is itself a very perilous haven. By contrast, you might value being Chinese more and yet still not worry much about the disappearing "Chineseness" of Chinese-Americans, because there is always China to supply more. But let me end with a question: What is "Chineseness" for you and for Chinese-Americans? Is it like "Italianness" for Italian-Americans or "Jewishness" for irreligious American Jews--basically an ethnic identity that is fading already and will inevitably disappear into the melting pot? Is the "cultural legacy" of which you speak not just an ethnicity that, in America, is doomed? Or is it something quite different, deeper, more valuable?