Just to well actually your well actually...

What you've described is how the base level mortgage-backed securities (MBSs) work. The tranches work because there actually exist mortgages that are at lower default risk (high home equity, well qualified borrowers, etc.), and the senior tranches are effective in capturing their underlying safety. What CDOs did was to take the lower, riskier tranches of MBSs from various sources and repackage them and divided them into tranches again. Then they got the ratings agencies to rate the top tranches of the CDOs as AAA as well. It's as if a teacher graded several classes and then took everyone that got a C or below from all the classes and then graded them on a curve again. And suddenly a lot of the C students became A students. It was outright financial insanity. Well, mixing a rocket/satellite company with a couple of also-ran AI outfits and the walking corpse of Twitter, and then calling the whole thing SpaceX and valued at $1.75T is a similarly level of financial insanity to me.