You can harvest ivory without killing the elephant immediately, if you are willing to put in the extra effort to do so. I expect the average poacher isn't.

However, a tusk-less elephant is a severely disabled elephant. You may not have shot it dead on the spot, but someone who de-tusks an elephant for the ivory has very likely killed it anyway.

Pretty sure you'd have to sedate the elephant. They like to keep their tusks!

Since and elephant would never forget losing its tusks (pun intended), and they learn from each other, I speculate that it would get harder to do this over time. ...Not to say that we couldn't, I mean, we have technology--they don't.

They won't accept only taking a portion of the ivory and leaving the rest for the elephant. The demand and the value for ivory incentivizes an "all or nothing" approach. That's why you see cases where elephants have been poisoned but their tusks remain in tact. It's because the poachers didn't have enough time to get whole tusks. The people actually poaching the elephants aren't the ones who make the big bucks, so they're likely taught by the traffickers how to obtain the tusks in order to maximize THEIR value.