I'd point out that the Romans themselves fought in the phalanx formation up until the Samnite wars. Samnites were the ones that first employed the maniple formation. After Roman phalanxes suffered a series of defeats in the Second Samnite war, Roman commanders decided to abandon the idea of phalanxes altogether and copied Samnite maniples. This new flexibility helped them win the war.

I wouldn't say maniples were ever unfeasible, it was just that the Greeks and the Etruscans never faced them on the field of battle (before Romans adopted them). When the Romans finally did, it turned out that maniples could not only stand up to the phalanx, but also completely outmanoeuvre and destroy it (a lesson that the Greeks and Macedonians later learned at the battle of Cynoscephalae).