Why did many people doubt the Anaconda Plan?

The concept of the "Plan Anaconda" was to strangle the Southern States in a tight spiral similar to the hunting strategy of the Anaconda snake.

General Scott’s idea was to set a large scale siege of all the Confederate States by deploying a maritime blockade of the Virginia, Carolinas and the Florida's harbours and coast to the East and a fluvial cordon of armed floaters on the Mississippi to the West.

The criticism were principally logistics. The Union did not have a fleet capable of enforcing the naval blockade, nor did it have enough barges adequate to patrol hundreds of river kilometres plus the myriad of streams and canals of the Mississippi Delta. The general wisdom was that, by the time the fleets were ready for the double blockade, the war would be over. For that reason a land conduct of the war was proposed to Lincoln and approved.