The reason for those "cones" is to create a friction that slows the synchros so that they can blend with the slotted collar that the shift fork is pushing.

Once that surface is glazed/smooth it isn't as effective as something that has a cut surface.

The synchro/brass ring/blocking ring inside surface is knurled for this same reason. It's not the teeth that slow the gears to provide a shift. That's called clatter.