That may be true for pristine or near pristine matching examples or historical items that have provenance such as a revolver owned by Teddy Roosevelt, but it is not necessarily true for shooter grade rifles. A lot of casual buyers actually prefer "pretty" rifles and are not really buying them for collectibility or authenticity but to shoot them (see the popularity of Mitchell's Mausers or Russian/Yugo captures).
For the most part, the military and whoever owned the rifle before did changes in parts, finishes, stock repairs and replacements, without regards to its future value as a collectible to maintain its status as a weapon. Mixmasters represent the typical shooter grade military rifle often do better at sale if they are cleaned up inside and out and perhaps even refinished and damaged parts replaced or restored which is what a military would have done if it was kept by them. Other than a few odd variants, WWI Enfields that went to India, got the Ishapore screw, had their volley sights removed or their cutoff, got refinished and restocked by India, etc. with an average bore and the usual gouges, paint in odd places, etc. value will remain consistent with those wanting to shoot an Enfield and not place it in the safe. Unless the person bubba's it by adding misplaced screw holes, butchering the stock and its hardware, causing mechanical failures by attempts at making a glass break 2 ounce trigger and deactivating/destroying safeties, cutting the issue barrel, or by rechambering in some weird obscure wildcat, mere cleaning and restoration, even to the extent of refinishing (if well done) can increase the value of a shooter at the low end.
Messing with a rare variant with historical and collectible value is one thing. Cleaning and restoring an average shooter grade rifle, not so much. In addition, scarcity is driving up shooter grade rifles as many are getting parted out anyway with a few still being sporterized. Even the parts, retain a value of their own which props up the low end of the market and most restorationists want parts in better condition (eg not rusted, pitted, etc. with a good finish) and the prices reflect that. Scarcity, condition, and demand set the price.
