This was all possible because XZ, despite being effectively everywhere, has one actual part-time maintainer who had other things going on in his life. That also means that there's nowhere near enough resources to redo the build system to some more secure alternative. If they had, say, 10 enthusiastic and skilled volunteer contributors with plenty of free time, they could do that, but then a new person appearing out of nowhere with a few helpful commits would never have a chance at being made an official maintainer or sneaking sketchy tools and code past the other maintainers.
Not that I'm blaming XZ or the real maintainer. Clearly whoever was behind this was looking for the weakest link, and if it wasn't XZ, it would have been something else. The real problem is the culture.
So I guess what this really means is someone at a big corporation making Linux distros should audit their full dependency tree. Any tool in that tree that isn't actively maintained, say at least 3 long-time active contributors, they should take over one way or another - whether that's hiring the current maintainer as a full-time remote employee, offering to buy out the rights, or forking and running under their own team.
I'm not necessarily super thrilled with that, but I guess it's the world we live in now.