velites were a relic of a system of Roman military organization based on economic class. By the end of the 2nd century BCE, those distinctions were increasingly nonsensical. In the old days, led by Brunt 1971, the common line was that the Romans were suffering manpower shortages in the middle to late Republic, especially on the "lower" end of the spectrum, because poor Romans were being pushed off their small holdings by slave-worked mega-farms. That notion is now very much out of fashion, attacked first by Rich in 1983 and then supplanted by Rosenstein in 2004. He argued that the lower classes began to actively avoid registering/volunteering for military service in the 2nd century BCE because they did not want to be sent to Spain, Rome's equivalent to Vietnam, which dragged on from 218 to 133 BCE, offered virtually zero promise of any exciting or lucrative booty, and was a dangerous, bloody, muddy, unpleasant slog for all involved. Between 133 and 106 BCE (the end of the Jugurthine War), the Roman army was undergoing major shifts, many of which are traditionally ascribed to Marius, but which were in reality more systemic. Increasingly, Roman citizens of means were finding it distasteful to serve as infantrymen. Young members of prominent families were finding new routes to power via the cursus honorum which precluded service, or else they were serving as junior officers (tribunes of the soldiers, increasingly) or as appointed lieutenants. By the time we get to Caesar, it seems that virtually all tribunī mīlitum were young men like this, and Caesar seems to have had a mix of young "apprentices" and older, proven soldiers as his lieutenants. Roman noble cavalry service was disappearing, and Roman junior commanders were proving unable to meet the challenges of emerging threats like the invasion of the Cimbri. Property requirements for service began to be relaxed out of necessity, and with the fading of those requirements, the old distinctions (like "velites") began to disappear. This process was accelerated by the Social War, 91-87 BCE, in the aftermath of which all male free-born Italians south of the Po became de facto Roman citizens (with some few exceptions). From this point forward, the typical Roman legionnaire became increasingly "Italian," rural, and of a lower socio-economic class. Roman cavalry service virtually disappeared after Sulla in favor of allied cavalry, particularly contingents drawn from Numidia, Celt-Iberia, Gaul, and the Germania. This is significant because auxiliary cavalry seem to have performed the traditional role of velites, only better. This, combined with increased use of the pīlum at medium to close range, made the role of the velites less important.

"velites" as a concept did not completely disappear. Caesar makes vague mention of soldiers in his army who do not seem to be legion heavy infantry or auxiliary cavalry. For instance, in Gallic Wars book 4, during the first landing on the shores of Britain, he mentions that he uses arrows (specifically sagittae, not generic missiles like tēla which could be construed as pīla) to push the Brits away from the landing beaches. I cannot recall off-hand any other mentions of archers or arrows in Caesar, however.