The Komarna explains in the opening words of the Notzer Hesed that none of what Mosheh Rabbeinu received from the mouth of God on Har Sinai could be transmitted. (Every word would take more than seventy years to explain)
On the day Mosheh Rabbeinu died, Yehoshua forgot thousands of halachot.
The codifying of the Tanach left out a number of books from that time period. The hashing out of the mishna left a tremendous body of baraita in its wake. The Talmud is but the smallest drop of the knowledge of the Amoraim, whatever could be committed to text, and whatever we haven't lost of that over the years.
This says nothing of the thousands, maybe tens (hundreds?) of thousands of sefarim that have since vanished from the world. (We're not even going into the secrets of the Torah, we have maybe seven extent chapters (Sefer Yetzirah) from the book of Adam rumored to have at least five hundred.)
Beyond all that, even of the Torah we have maintained to this day, volumes seemingly without end, so much of it has transitioned from Torah with practical application (only in the most literal sense, chas v' shalom that someone should think it needn't be learned) to Torah for its own sake. So many practices and Halachot are no longer performed or held today, either from chumrot, kulot, or different minhagim.
But really, if you think about it, it isn't so shocking.
There are some who might look at all this and see evolution, survival of the fittest, or even a readjustment of norms to meet the expectations of modern society, but I believe that is foolishness at best.
The reality is that yes, it's entirely organic, but in a way our everyday short-sighted view couldn't begin to comprehend.
When we look at the entire Jewish people as comprising a single body, one organism that persists throughout time as well as space, we recognize the deeper meaning of the identification of these generations as the heels--the ends of the feet. It begins to dawn on us, from this perspective, that just as there are veins, arteries, sinews, nerves and more that connect the feet with the rest of the body, there are many more veins, arteries, sinews and nerves, that don't quite reach the feet, many, if not most, never even come close.
If Torah is an integral part of the Jewish body, the life force of this body, then the abundance of Torah centralized in the head (Mosheh Rabbeinu and his generations) and the chest (the Mishna/Talmud and their generations) makes the Torah in the feet almost negligible by comparison.
So, rather than being shocked and appalled by what has become of our Torah, we can begin to understand the role and part our Torah in our day plays in the greater timeless body of Israel and Torah.