In MM6-8 I usually use my Archer to disarm traps. I can't remember who I used in MM9, but I think it was my lich. In MM3 there is a way of getting the thieving skills for any character, and I find that s sorcerer makes an excellent thief once he's got the skill. In World of Xeen (MM4-5) I have to use either a robber or a ninja – I've not been able to find a magic user who can learn to diarm traps and pick locks, so the last time I played, my party was ninja, ranger, paladin, archer, cleric and sorcerer. Like you, I normally use my cleric as the trader, and also (eventually) for repairing. I never use any of the NPC repairmen because none of them can repair everything.
Turning to operating systems and upgrades, I've found that upgrading to a new operating system is not always a good idea – or to put it the same way you did, "if it ain't broke, don't mend it." My experiences so far have been that the operating system installed on a machine when you get it, together with its regularly supplied official upgrades, have worked well (with the well-known truly abominable exception of Vista). But Microsoft offered a free download to upgrade Win7 and Win8 to Win10, and of all my friends and acquaintances, only one was able to apply the upgrade without problems. Most of them at some point met a BSOD coupled with a "Something happened" error.