That's like saying driving from NYC to Sacramento is just a "Stepping Stone" to driving to SF. You've done most of the drive.

To get 1kg of U-235 requires 1.11kg at 90% purity, 1.67kg at 60% purity, and 140.6kg at natural 0.711% purity.

You only get to 60% on the road to 90%. At 60% it has no other useful purpose.

I read an interview with a nuclear physicist the other day who said "If you can do 60%, you've nailed the technique, so 90% is not difficult anymore."

Besides, there is no non-military need for enriching uranium beyond a few percent, so it's very clear what those 60% mean.