A lot of people did adopt Alexander's style of warfare, or emulated to varying degrees in the time after Alexander's death. The pike phalanx, with its sarissa spears was the dominant method of warfare in the Eastern Mediterranean for over a hundred years after Alexander's death.

First and foremost, this style of warfare was the mainstay of the states directly descended from Alexander's empire - Macedonia, Seleucid Syria, and Ptolemaic Egypt. It was also emulated by several Greek City-states, including Pergamum. Smaller Greek Kingdoms also adopted the tactics - such as Epirus, and Pontus.

In any case, your original premise is flawed. The Romans arose to prominence after Alexander's death. Their original encounter with the Pike Phalanx was in a war against King Pyrrhus of Epirus, who triumphed over the Romans twice before being defeated a third time. The Romans then defeated the Macedonian Phalanx, and later the Seleucid Phalanx in subsequent wars - Alexander's way of warfare met the Roman way and was defeated, soundly and consistently.

The exact reasons for this are probably a discussion for another time, but the pike-phalanx of Alexander's day was an unwieldly beast, powerful from the front but inflexible and difficult to use.

Your initial premise seems to assume that, out of ignorance, everyone suddenly stopped using Alexander's tactics and that the Romans subsequently 'rediscovered' the usefulness of the spear. This simply isn't the case, the Romans were using spears the entire time, just not the 18-foot long sarissa pike, which was useless out of a very specific formation, and method of war-making.

The short answer is that the Romans used spears - practical spears - when they were needed. But for their style of warfare, the gladius was a reliable and war-winning weapon by no means inferior to the Macedonian style of war.