We discussed this when I was a Chinese history student several years ago. There are a few points to consider:

• There were multiple voyages of different sizes, but the smaller ships in each voyage were unremarkable. The so-called "Chinese treasure ships" were the only unusual vessels (historically, anyway) due to their great size.
• The size of the ships is widely considered to be exaggerated, since ships of comparable size did not exist until iron-hulled steamships started being built. For this reason, it is not so remarkable that the ships would not have been preserved.
• There were only ever 60 or so of the great treasure ships.

With all of this in mind, the fate of the treasure ships is essentially unknown. Records indicate that they were stored in harbors after the last of the voyages until eventually they sank or were scrapped. Some believe that the ships were burned afterwards as part of the policy of Haijin, although in my opinion this is not a good theory.

What does not seem to be in doubt is that after 1433, the treasure fleets stopped sailing, and Ming China turned away from the sea. While ocean trade between China and its neighbors continued, the navy never really controlled the waters, and as a result China's maritime trade suffered heavily, not to mention that China was left weak and open to wokou attacks in the coming centuries.