Easy to call Earnur a fool, but he led the army that destroyed Angmar.
If Aragorn was the greatest in his line since Arvedui, then Boromir was considered to be the like return of Earnur.
The dispute between Gondor and Arnor about whether Arvedui's claim was good was based on a dispute over two things:
1. Whether the line of descent could pass through the female. Arvedui said yes, Gondor said no.
2. Whether Isildur intended for Arnor and Gondor to be divided. Arvedui said no, Gondor... kinda didn't reply because they were losing the argument, and stopped talking to him.
Tolkien makes it clear that Arvedui was in the right- the descendant of the High King of a Unified realm and with every right to take that throne- and then choose to leave someone to rule in his stead if he so wished. Because Gondor chose badly, the chance was lost for the the great rebuilding of the Kingdom of man to happen then, instead of 1000 years later under Aragorn.