A crash into what alternative? If people are trading away their USDs, what are they trading them for?
Why does there have to be an alternative for the USD to implode? Why would a lack of an alternative sustain an unsustainable system which appears to be cracking under it's own weight right now? The US govt being forced to issue debt at unsustainable interest rates is the only cause of death you need if people don't think they'll get their money back. They will figure out an alternative in that situation.
USD's supremacy isn't just from being the "safest" asset you can hold but also that you need it to buy energy from the middle east, since they agreed to only trade in USD in exchange for US military protection. Also because everybody else uses it. I doubt BRICS will be the alternative though, even if it is a half hearted attempt to get around SWIFT and USD.
And right now it seems to be gold. China has been dumping US treasuries since 2020 and moving heavily into Gold. France did the same. Althought thats not a viable alternative... there simply isn't enough Gold. In an ideal world we would move towards a Keynsian basket of goods as the international unit of exchange instead of a domestic currency but we'll probably pick the dumbest, least painful in the short term, most painful and debilitating in the long term answer... nobody plants trees anymore.