They were replaced with the vicious and brutal who always appear in such environments. Volunteer fighters from other Slavic countries have entered on the Serb side, and volunteer fighters from Muslim countries have entered on the KLA side. The choice was not now or never. The now part was a rather cold calculation by Mr. Clinton that this would be the right time to buff his image after the impeachment process. Yes there were missed opportunities in the past, and there would and will be opportunities in the future.
Diplomacy often requires planning, consistent pressure, and patience.
None of which has featured in US policy. NATO did not win. The entire Balkans is in a worse state now than when they started. Perhaps the only good thing is that enough resources will flow into the region to reconstruct a better place. They could have done this for a lot less money without the bombing.