This is such a good point and something what seems to have gone missing over time, due to every new expansion release coming with a new tier of gear (as requested) it's just so happened to be a by product of the issue you've highlighted Darren, thus making the whole previous area useless once the next expansion is released.
Back in the day Slave Cave was relevant because you could easily farm Benedictions, so players would hunt that cave specifically for that. I know benedictions are a gamepoint thing now and are totally fine, however it is just an example of how that cave had its unique purpose.
I do think a rune-dropping cave would be good, or somewhere in a cave or even a boss KR, some runes can be mined from a node which has a long and/or random respawn rate. These type of things probably have to be reviewed pretty frequently because as the game progresses and players stats develop with further expansions you would end up with a low risk, high reward easy spot to farm, which isn't what I'm suggesting.