Nonsense --- is this some kind of Possibility Thinking? (Chip)
[Joshua 1:12-15] And to the Reubenites, and to the Gadites, and to half the tribe of Manasseh, spake Joshua, saying, Remember the word which Moses the servant of the LORD commanded you, saying, The LORD your God hath given you rest, and hath given you this land. Your wives, your little ones, and your cattle, shall remain in the land which Moses gave you on this side Jordan; but ye shall pass before your brethren armed, all the mighty men of valor, and help them; Until the LORD have given your brethren rest, as he hath given you, and they also have possessed the land which the LORD your God giveth them: then ye shall return unto the land of your possession, and enjoy it, which Moses the LORD's servant gave you on this side Jordan toward the sunrising.
[Joshua 13:8-12] With whom the Reubenites and the Gadites have received their inheritance, which Moses gave them, beyond Jordan eastward, even as Moses the servant of the LORD gave them; From Aroer, that is upon the bank of the river Arnon, and the city that is in the midst of the river, and all the plain of Medeba unto Dibon; And all the cities of Sihon king of the Amorites, which reigned in Heshbon, unto the border of the children of Ammon; And Gilead, and the border of the Geshurites and Maachathites, and all mount Hermon, and all Bashan unto Salcah; All the kingdom of Og in Bashan, which reigned in Ashtaroth and in Edrei, who remained of the remnant of the giants: for these did Moses smite, and cast them out.
This used to be Moab also before the Amorites killed them all and took it for themselves. Moses then did likewise to the Amorites. This is why the land was still called Moab until the Assyrian dispersion 700 years later. Ruth was an Israelite resident of this Moab.....not the Heathen Kingdom of Moab.
Well some of them, along with Ruth, must have survived because Ezra saw them in the Land of Israel as late as the 4th century BC:
"Now when these things were done, the princes came to me, saying, The people of Israel, and the priests, and the Levites, have not separated themselves from the people of the lands, doing according to their abominations, even of the Canaanites, the Hittites, the Perizzites, the Jebusites, the Ammonites, the Moabites, the Egyptians, and the Amorites".[Ezra 9:1]