Unfortunately, the Supreme Court doesn't take on "what if?" cases, and so there is no way to hear a challenge over this issue without electing a president who chose a VP who has already been president twice, and then the opposition party suing over it. And even then, they probably couldn't meet the "actual case or controversy" rule without the president dying and the VP needing to assume office as president. At which point, the VP-becoming-President would be at the mercy of five unelected lifetime-appointed judges, who may or may not decide to rule based on their own political biases (e.g., "like RBG") or blackmail (ahem) or a textual interpretation ("elected") or an "intent" interpretation (no permanent presidents-for-life like Putin).