No, he was not.
William III's heir presumptive would be his sister, Princess Sophie of the Netherlands (1824-1897).
Next after Sophie was her eldest son, Charles Augustus of Saxe-Weimar-Eisenach (1844-1894).
Then the eldest son of Charles Augustus, William Ernest of Saxe-Weimar-Eisenach (1876–1923).
Given his mother's age and the general preference for male inheritance, Charles Augustus was the
de facto heir presumptive to William III before 1880 and to Wilhelmina after 1990. His succession was considered very possible, and he went to the trouble of becoming fluent in Dutch, just in case. He was allegedly popular with the people of Saxe-Weimar-Eisenach, and very probably would have been accepted as King.
However, Charles Augustus predeceased his mother, and after her death, William Ernest was indeed the heir presumptive to Wilhelmina. He was not popular; his personal life was unsavory, and he was suspected of driving his first wife to suicide. Also, his succession would have brought the nation under excessive German influence. There were moves in the Netherlands to remove him from the succession. In 1922, Dutch law was amended to restrict the succession to descendants of Wilhelmina.
However - Wilhelmina was William III's only child, and she had only one child (Juliana, 1909-2004). Juliana had no children until 1938, when the future Queen Beatrix was born. (Juliana had three additional daughters.) So there was a very long "bottleneck" for the House of Orange-Nassau.
Several possible PoDs appear.'
- Wilhelmina dies of typhoid circa 1905. (OTL she survived.)
- Wilhelmina and Juliana die in a failed childbirth in 1909. The Dutch proclaim a republic to keep William Ernest out.
- Wilhelmina miscarries in 1909 (as she did on three other occasions), and remains childless. At her death in 1962, rather than have an obscure German cousin succeed, the monarchy is abolished.
- Wilhelmina miscarries in 1909 (as she did on three other occasions), and remains childless. Wilhelmina is killed in a train crash in 1917 (OTL no one was killed). Germany insists on German cousin succeeding (William Ernest was expected to abdicate in favor of his cousin Heinrich XXXI of Reuss-Kostritz). Instead, the monarchy is abolished.
- Juliana dies without children. Under the 1922 law, only descendants of Wilhelmina can succeed. At Juliana's death (or Wilhelmina's, if Juliana predeceases her), the monarchy is abolished.