A different time in history is important, but that doesn’t change the issue of openness versus closedness. In a time of nation-states and borders and tight control, there are laws about the movement of one people into the territory of another.
Moreover, Ruth had marriage rights in a land not her own by virtue of her marriage to Naomi’s son. Those rights were honored even though she were a foreigner.
Ruth didn't cross the border with a few thousand Moabites claiming such "rights" in Israel, so I think your analogy begins to break down. The equivalent situation would be if an American family moved to Mexico, the sons got married but died, and one of the widows came back into America legally with her mother-in-law to assist her in her old age, adopted into the culture ("Your people will be my people and your God my God.") and later earned her citizenship legally after marrying an American.
I don't think we'd be having this conversation if that were the situation.
Shalom