Ah, I see what your problem is. You're cheering for "quantum computers" because they sound cool and science fiction-y. While quantum computing theoretically provides ways to very rapidly solve certain sorts of problems, it doesn't just magically solve all problems. Even if the algorithms that run our universe are well suited to quantum computing, they still run into the speed and memory issues that classical computers do, they would just run into to them a little later (although even that's not guaranteed - the speed of the quantum computer depends on the number of entangled qubits, and for the foreseeable future, it will be easier to get more computing power by adding to the size of our classical computing clusters than ganging more small sets of entangled qubits together). The accurate statement you should be making is that modeling many worlds with a significant number of branches or scope is intractable using any foreseeable computing technology.