Funny thing about this narrative. The idea of the Roman soldiers fighting overseas and thus losing their family possessions because they can't work their plot, thus forcing them to migrate to cities causing all sorts of problems, is largely one derived (at least in the current form) from Theodor Mommsen, easily one of the greatest and most influential classical historians of all time. Mommsen's life coincided more or less to the nineteenth century, and naturally he viewed his studies through the dominant social theme going on outside his window--the social turmoil caused by the Industrial Revolution, rapid urbanization resulting from that, and the creation of a tumultuous urban socio-political scene. His narrative is essentially Industrial Germany in a toga.

This is not to say he was entirely without justification, as plenty of ancient authors bemoaned what they saw as the death of the Roman freeholder. Their words are useful, they were there, and to an extent we need to trust them--but only to an extent. In this particular case, the landscapes empty of good, yeoman Roman farmers directly contradicts the archaeological evidence, which points towards fundamental continuity, albeit with intensification, of landowning patterns. Furthermore, the literary evidence itself isn't fit to bear such a narrative, as the authors don't talk about the sort of urban overcrowding resulting from rural economic refugees you would expect (urban crowding is embedded in highly paradigmatic passages) and besides, a premodern city almost certainly couldn't support a vast slum population. The vast slave run latifundia often read about have far more to do with the New World plantations, which developed out of highly unusual circumstances, than anything in ancient sources.

So the narrative is not really supported by ancient evidence. It isn't entirely wrong, as there was almost certainly some sort of wealth consolidation beyond those earned from provincial conquests, and this may have led farmers to fall under the sway of landholders in tenancy arrangements leading to deleterious social effects, but rumors of the death of the Roman peasant seem greatly exaggerated.

There are a few things here that I would disagree with. The first part is that lands wouldn't be gobbled up unless there wasn't anyone to support that land. This land confiscation would have happened when soldiers would die and their families were unable to support the land. This didn't really happen until the Third and Second Century BCE when Rome was more involved in military conflicts that would require more time than the standard war season when either families would be unable to complete with wealthier land owners that employed slaves or unable to support the land due to the loss of family members by either excessive time away or death. A further thing to mention is that while there isn't pay, there is loot, and especially with some wars (such as the Third Punic War or the sacking of Corinth), there would be a massive influx of wealth not just for the commander but for the soldiers.