While you are mostly correct, I would think that the fact someone intermarried is a good indicator of weather or not Judaism is relevant or engaging to the person who "left the fold".
Meaning, the fact that someone marries out is a sign that they didn't find Judaism engaging, and thus is is a bad thing for the Jewish people.
It's really hard to tell if someone is affiliated or engaged in "their Judaism" otherwise.
Frankly, I'm not sure this argument can be applied so universally any more. My personal experience is precisely the opposite- I have four sets of aunts and uncles and my parents were the only ones that married Jewish (mostly by accident). This resulted in my parents being the most relaxed about whether or not to observe any traditions, holidays, or provide anything remotely resembling a Jewish education. Everyone else, one way or another, actually had to make a decision about Jewishness.
Two of my aunts and uncles are active in their Reform shuls and have raised their kids far more Jewishly than I. Another one, after many years of ambivalence, has started going to High Holidays. The last one converted to Lutheranism after feeling that her husband, who had ostensibly converted (not sure on the details) was still snubbed by my grandfather. Today her daughters are nominal Christians.
Until my teens, I had no Judaism, and only a very slim Jewish identity, mainly defined as otherness. In over ten years I have come a long way, and while I am far from Orthodox (or particularly observant of any mitzvot), I am one of the most Jewishly educated people in my family. I am lucky enough to have found a woman that is interested in the traditions, rituals, and philosophies of Judaism but feels it would be inappropriate to convert as she does not really believe in the theological details (as, on many days, I don't, either).
I am engaged with my Judaism, and am motivated to perpetuate it with my eventual children. I know that she is, too. My Judaism will be passed down because it is an integral part of me. The fact that I am in a long-term relationship with a non-Jew does not change that. Had I been raised observant, and conditioned to believe that a major tenet of Judaism and the Jewish home is that Jews must marry Jews, then I would agree with you that the mere act of marrying out might be a strong indicator of lack of Jewish identity. But there are millions of young Jews today who have not been raised that way, and do not accept this barrier as the defining line of who they are. That makes it outmoded. Fifty years ago, probably. A hundred years ago, a very good possibility. Not so now.
We will see what direction the statistics wind up leaning-- certainly, in terms of pure birth rates, the Orthodox have a very strong head start. But whether the non-Ortho movements wind up shrinking or not does not mean that they will become extinct, as the Orthos triumphantly proclaim so often.
Intermarriage does not equal "Jewish death" anymore, (if it ever did-- see Moses and his kids) and I would question the spirit of ahavas yisroel or klal yisroel of those who try to denigrate or discourage those Jews who stay involved with their people, because they "aren't good enough." These folks should be celebrated and encouraged, not pushed away as if their imaginary disease might infect the others.
There are plenty of reasons for Orthos stigmatizing non-Orthos (I don't agree with them, obviously) but "intermarriage destroys the Jewish people" isn't one of them. Not anymore.
As for "hard to tell if someone is affiliated or engaged with their Jewishness"-- since when did laziness become an excuse for bad methodology? Someone might be intermarried and lead their own indie minyan. Someone might be intermarried and be heavily involved in Jewish culture. Someone might be intermarried and study Talmud regularly. Why should their Jewishness be disqualified because of their spouses' lack of conversion? (I accept that there may be a range of "identity"- that some facets, or identification points, might be more shallow while others more in-depth, but that doesn't negate that it's there.)
Jewish identity can occur in a multiplicity of ways-- and because that is so, the idea that there is any absolute breaking point is not really applicable. Slippery slope, but that's what identity is.