Rashi: They were pointing out that, unlike a sale, where the land reverts to the original owner in Yovel, and where the problem would not exist, inheritance does not go back in the Yovel. ¹

Seforno: The Torah is referring to a later stage, when members of a certain tribe have sold part of their inheritance to members of another tribe before it has been captured, they will not hesitate from going out to fight for it, knowing that, when Yovel arrives, it will be returned to them, which is not the case in the current situation, where, in the event that the B'nos Tz'lofchad would marry into another tribe, the members of Menasheh would refrain from fighting for that territory, since it will not be returned to them - thereby causing a loss to the national inheritance.

Rashi #1 (citing R. Yehudah in the Sifra): It implies that, at some stage, the Yovel cycle will be broken. ¹

Rashi #2 (in Rosh Hashanah, 3a): "ve'Im" here means (not 'if' but) 'when', because, since the Yovel is not a Safek; it is bound to occur. ²

Oznayim la'Torah: It is possible for the Yovel to become Bateil if two tribes become mixed ³ - if for example, the majority of inhabitants in Binyamin are from Yehudah. Consequently, the heads of the B'nei Gil'ad were expressing their concern even assuming that only one or two daughters would inherit their fathers from different tribes, and the Yovel would remain intact.