What critical refinement did early derailleur designs need besides better chain throwing?

A reliable system to tension the chain once it moved to the smaller cog.

Although Paul de Vivie patented the derailleur concept in 1895, initial designs suffered from significant functional shortcomings that kept them from widespread adoption for many decades. Beyond the challenge of effectively 'throwing' or moving the chain onto a different sprocket, a second critical mechanical hurdle had to be overcome. This involved ensuring that once the chain successfully moved to a smaller cog, there was a reliable means to maintain the necessary tension. Insufficient tension would lead to the chain slackening, potentially causing it to drop off the sprocket entirely, rendering the gearing ineffective until manually readjusted.