Barring federal interference, couldn’t the DA simply choose not to even present to a grand jury?
Absolutely. The DA can present or decline to present. He can then present a case that heavily favors indictment or a case that heavily favors no indictment, a “no true bill” ruling. It completely up to him.
That would leave Wilson up in the air, as some future DA might choose to bring it up again. He's in a better position (legally and PR-wise) if the grand jury sees the evidence and "no bills" him.