The short window when he rose to the top was depleted in all regards and by all evidence... old ass fighters who can't perform close to their best, smaller fighters trying to make a paycheck, and some white dopes of middling talent and less skill. ATG for career timing and management.
I'm not a big fan of that weak era argument. Surely, Marciano's era wasn't the first half of the golden seventies (Ali, Foreman, Frazier) but which one was...? Even if it was a weak era... what's up with other Champions? E.g., Jack Johnson, did he fight Langford, Jeanette or McVea as Champion? How active was Jess Willard? I think Jack Dempsey is sometimes viewed less critically than Marciano, but did he fight Wills or Greb...Did Joe Louis, within his long career, defeat way better boxers than Marciano? Or Floyd Patterson (no Folley or Machen but Rademacher and McNeeley)...60s Ali vs. old Williams, old Folley and London... Larry Holmes in a not so strong era as well missing some "nice to haves" like Thomas, Dokes, Page or Coetzee... The 90s were great but still no: Lewis vs. Bowe, Bowe vs. Tyson and Lewis vs. Holyfield was late and Lewis vs. Tyson was waaay too late... And Klitschko's opponents were also not the greatest thing since sliced bread... I just mean you can criticize pretty much anyone if you want to. Marciano's opponents’ choice certainly wasn't perfect for today's boxing fans, but I think one can criticize most others even more. Overall, I think the average quality of Marciano's title or quality contenders wasn't weak! They were just small by today's standards, but it was another time, and I don't care about this imaginary "H2H" stuff (look how short some of Ezzard Charles' title contenders were too...) Another point is that his career was rather short, and he retired before he significantly faded (which was smart as well). And finally, I think the difference to many others is that he didn't have any really great opponent. I mean HW-ATGs like prime Louis, Ali etc. or at least younger versions of Charles or Walcott... But as already mentioned, I think the average quality was decent. To me he is just a legend, easily heavyweight Top 10 of all time and a symbol for determination and toughness.
His best 5 wins Charles x 2, Walcott x 2 and Moore are great wins. Not many have 5 wins better than that. For example Lewis best 5 wins are Vitali, Holyfield, Mercer and Maybe Rahman and Tua. So the cream of Marciano’ s resume is very good and better then Lewis’s, but Lewis fought a lot higher ranked fighters coming up as a pro, but he had a long amateur career also.