That contention essentially means you can tear out the Dinei Mamonos sections of Shulchan Aruch and replace those pages with your State’s local legislation on monetary matters. That the Torah’s laws (as elucidated in S”A) on yerusha are null and void and completely without meaning or effect, as you contend that gentile law on inheritance completely nullify all the halachas on yerusha. It is completely absurd.
And Rav Eliashev has a teshuva that states that the halacha of Dina D’malchusa does not include monetary matters between people (but, rather, only governmental matters such as taxes.)