Very solid justification for your argument. Clearly Frazier had better quality wins.
The undefeated record is what gives Marciano the edge, and it's not simply because he never lost. Certainly the 49-0 record stands out and helps his legacy. Losses also give us perspective into what type of fighter or strategy a fighter is vulnerable to. Frazier for example, we saw what Foreman did to him and we saw a post prime Ali outbox him twice.
Marciano is in a position where a further level of speculation is needed to determine what potential flaws could be exploited to beat him. Gun to my head, I'd agree that Marciano loses to 1971 Ali and gets knocked out by Foreman. However, you can't actually bring up specific fights in which he was knocked out early or lost a decision. It creates a less solid basis in which to speculate on Marciano losing.