Just like with [any single-author creator-owned work of finite duration], Japanese authors prove [as a poor straw man] they can write a way better Superman [the world will barely know] than what we've got nowadays.
This is and always has been a ridiculous comparison.
Mainstream comic characters are corporately owned and exist in a collective consciousness spanning close to a century for Superman. You have a myriad of takes, tastes, and trends over that time. Civil rights and desegregation is newer than Superman. The state of Alaska is newer than Superman.
No mangaka has to contend with expectations on an eternal emblem, no mangaka is constrained by editorial or audience to present a classic again and again as somehow unchanging but always fresh and new, no mangaka is holding back for their own creator-owned works because this
is their creator owned shot at glory.
Just because they're both sequential art doesn't make the expectations on both the same. Between 1986 and 1994, Superman featured in 541 issues. 5.6 books a month, full-color at a faster pace than the most relentless mangaka, all while trying to maintain continuity and coherence across different creator visions, teams, and titles. That's just a small 7 year slice out of 77 years.
At best, the comparison says, "Let someone do a long run immune from interference and greatness
can arise..." not "
does arise" because even with the advantages of weekly storytelling, no expectations, creator-controlled continuity... the vast majority of manga is not noteworthy. Pointing to an exception as proof of prevalence is pointless.
Yes, most Superman stories aren't great, but neither are most comics or most manga... otherwise your definition of "great" is so broad as to be pointless. However, it's a machine that's good at what it does.