At the risk of exposing my obvious naivety, my guess is that TSLA is a proxy purchase for many people for SpaceX. I won't be surprised if SpaceX becoming public causes TSLA to tank.

Or maybe it's all because of index funds. What bothers me most about that is that if TSLA tanks, so does a big chunk of the S&P 500 and therefore my 401(k). Hrmph.

I suspect you may be right. My guess is that Telsa will be bought out by SpaceX so they can cover up loses from various parts of the company, namely X and xAI.

Like security backed bonds but on a company scale.

With SpaceX having loads of government contracts, they become more immune to failure via odds of a bailout.