Even if the amount of DNA only adds up to 20% of their mix (the average), it's still very significant.
Bretons are Celts just like Gauls, whom you missed. You also missed other Celtic groups. And they (Celts) were the majority before invasions, still the majority.
Indigenous doesn't mean minority, it means the original inhabitants.
That is, until the Islamic raids started, the indigenous groups took losses, but were not nearly as transformed. Is that not accurate?
Arabs were the highest-prestige ethnicity within their empire. But they were numerically insignificant. You have "Arabs" and "Copts" in Egypt today, but most of the "Arabs" are Copts whose ancestors decided they wanted more respect. Same thing is true farther west, where the "Arabs" are Berbers.