Friends:
All of this sleeving v. monobloc stuff is interesting to read but bloody immaterial to the original issue as posited, i.e., why would a sleeved gun be of less "value" than an originally-barreled gun? The answer to that does NOT lie in the fact that a properly sleeved gun is less strong or safe than an original barrels. Indeed, many sleeved guns are actually more robust than the original barrels may have been.
Sleeved or monobloc, they are both two-piece assemblies. Each proven safe and mechanically reliable.
Thus, the most reasonable answer to the question of value is that the sleeved gun is simply no longer "original." Even though it may be functionally, and practically superior. Perhaps even aesthetically.
I look for this thread to go on for at least another six weeks.
Best, Kensal
