The losses fall on bondholders now, but it does make it harder for SpaceX to raise money going forward. And if they actually slip into junk territory, some institutional investors will be forced to sell (mandates only allow investment-grade), pushing prices down and yields/spreads up even further.

That can snowball: wider spreads → higher borrowing costs → more stress → wider spreads. The existing bonds' coupons are fixed, so the real bite is on future issuance and refinancing.

Lots of capital-intensive companies (SpaceX is definitely in this category) lean heavily on debt markets to fund ongoing investment and roll over maturing debt, so losing cheap access is a big deal.