I thought the number might be a little higher given we know the flintlock system was used on the frontier even into the very late 19th century, albeit primarily by Native Americans.
The rifles used on the frontier had a very low survival rate period. Lee Burke in one of his papers on the Carolina Gun estimated that 5 years was a reasonable maximum life of an Indian gun in the south during the 18th century. This would likely include the gun being repaired a time or two. Guns on the frontier were tools and tools were used hard 'til they were used up.
The Northwest Trade Gun is the most common trade gun existing today, but that is because so many of them were made over such a long period. The surviving guns are a very small percentage of the total that were made.
I've not seen any statistics on converted versus original flint for the surviving trade rifles associated with the western fur trade, but nearly all of the ones that appear frequently in publications have been reconverted back to flint including the three J. Henry trade rifles (the Lancaster, the Old English, and the New English) in the Museum of the Fur Trade.
The rifles that have survived in their original flint configuration were probably put away for one reason or another fairly early and no longer used. This may have been because they were no longer needed and put away and forgotten, or they were family heirlooms and saved as they were, or they may have had some historical or collectible significance and preserved.