Upvoted for witty reply.

I didn't consider the moral implications. They are complex.

If you think about it though, the great future promise of the Singularity for humans is some type of uploading into designed virtual universes (the heaven scenario).

And in our current (admittedly) simple precursors, we have no compunctions creating sim worlds entirely for our amusement. At some point that would have to change.

I imagine there will probably be much simpler techniques for making safe-enough AI without going to the trouble of making an entire isolated sim world.

However, ultimately making big sim worlds will be one of our main aims, so isolated sims are more relevant for that reason - not because they are the quickest route to safe AI.