I believe the new transfer case will last longer than the original, because the steel will not fail as rapidly as the aluminum does on the originals. As for turning off the 4matic system, I wouldn't recommend that, (though I'm not a tech), because there are other systems, such as the self-leveling suspension that are intertwined with the 4matic system, so to disconnect one is to disconnect them all. If you do a search regarding the 4matic, you will find that this subject, and disconnecting the system vs. not, has been discussed previously.
As I am sure you well know, when the 4matic sytem is working, there is no better 2/4wd system out there, even now, ten plus years later. The 4matics do have their headaches, but they are worth it in snow country!

1962 MB 220SEb "Fintail" Sedan
1992 MB 300TE 4matic Wagon
1995 Subaru Legacy AWD Wagon