So I got a 27" iMac a couple months ago, which i love, but what i can't stand is the inability replace the HD, should it fail.
Whatever led you to believe it is not replaceable is not correct.
It is replaceable, though not without some difficulty. Using an external HD in an effort to extend the life of an internal component responsible for much of the iMac's performance doesn't make a lot of sense.
A major cause of drive failure is intrusion of environmental contaminants that is guaranteed to eventually occur due to thermal cycling or atmospheric pressure changes, even if you were to never use the drive. Unless you're going to use an iMac in a clean room, nothing will prevent a tiny smoke particle from eventually penetrating its case, finding its way between a platter and head, and permanently ruining it.
Use your Mac the way it's designed to be used. Hard disk failures occur, but it should be expected to last for the economic life of your Mac, which historically has been several years.