This theory seems plausible. The only explicit criticism I have found is from a paper by Graham Priest and some collaborators which claims in a footnote that the relativity of truth, from which the relativity of existence is derived, is dangerous considering we no longer have statements or facts that are simply true. I'm not sure this is necessarily a problem, and Yagisawa already addresses a similar concern in footnote 30. Under Yagisawa's theory, there is no failure to explain what truth or existence is--they are well-defined relations. Thus and infinite regress based on relativity does not imply a vicious infinite regress; we are not defining existence based on these relations, existence is the relations. I am curious what others think as this theory is important for explaining certain aspects of Yagisawa's Extended Modal Realism, which could be a useful analytic tool if proven true.