From what I can tell on the outside, the MIRI approach seems to be: (1) find a practical theory of FAI; (2) design an AGI in accordance with this theory; (3) implement that design; (4) mission accomplished!

Yes, dear, some of us are programmers, we know about waterfalls. Our approach is more like, "Attack the most promising problems that present themselves, at every point; don't actually build things which you don't yet know how to make not destroy the world, at any point." Right now this means working on unbounded problems because there are no bounded problems which seem more relevant and more on the critical path. If at any point we can build something to test ideas, of course we will; unless our state of ignorance is such that we can't test that particular idea without risking destroying the world, in which case we won't, but if you're really setting out to test ideas you can probably figure out some other way to test them, except for very rare highly global theses like "The intelligence explosion continues past the human level." More local theses should be testable.

See also Ch. 22 from HPMOR, and keep in mind that I am not Harry, I contain Harry, all the other characters, their whole universe, and everything that happens inside it. In other words, I am not Harry, I am the universe that responded to Harry.