This is certainly true for some applications, but originally when Amdahl's law was formulated, people made estimates based on e.g. 95% of an application being p

This is certainly true for some applications, but originally when Amdahl's law was formulated, people made estimates based on e.g. 95% of an application being parallelizable, and thus fairly rapidly reaching a point of diminishing returns. In practice, however, there's often a simple reformulation of the problem that can result in a much higher percentage, and Amdahl's becomes less an obstacle than it would first appear. There are still many problems that scale more or less linearly with the number of processing units and are "trivially parallelizable".