Aha! True, if I were to open the Quran and read it for the first time, I will NOT interpret it as it was 1400 years ago.
But I think you have missed a point. Now I am not claiming that Muslims interpret the Quran 100% as the prophet used to. But the interpretation is more or less very much presreved...why? Because we have "Tafseer" books. "Tafseer" means interpretation. For perhaps every century there are "Tafseer" books, even the first century or so of Islam, the "Tafseer" was preserved via Hadith (which is also a very elaborate system of preserving the Prophet's sayings, really impressive tbh). This is why many scholars consider the invasion of the monghuls on Baghdad in 1258 one of the most catosrophic events on Islamic culture...The Euphrates and Tigris turned black from the ink of the books thrown into it....The world have lost a LOT, not just Muslims from that incident. However, other libraries around the Muslim empire helped keep all these sciences (including Tafseer and Hadith) intact.
Another thing that you might have overlooked, is the "Ijaza" system itself. Through this system the interpretation of the Quran was transferred from generation to generation. Of course it won't be as precise and perfect, but you would be amazed how immaculate Muslims become when it comes to the Quran and Hadith
Now interpretation is one thing, and execution is another, sadly these days Muslims (including I), are as far from what we preach as possible....