Several problems with your counter-hypothesis: 1. It is inconsistent with historical records. There's no evidence showing that the ancient Gauls, Celts and so

Several problems with your counter-hypothesis: 1. It is inconsistent with historical records. There's no evidence showing that the ancient Gauls, Celts and so on are of the same nationality as that of Romans. Their Y DNA are unlikely the same. 2. It assumes the invading Franks, Goths, and so on have different gene from the general public that they ruled over, i.e. Goths and Franks were not R1b, but there's no evidence to that. There's no evidence that the European medieval noble class were not R1b. 3. It is inconsistent with the common phenomenon of an invading ruling class's Y DNA quickly displace the ruled ones. A recent example is south American. R1b becomes the majority in a little over 500 years, displacing the original Q DNA. 4. The timeline does not work. R1b is a new gene originated about 10000 years ago in Asia. There's no way they are the one first colonized Western Europe. People with I, G and other far older genes have been there for a long time when R1b shows up.