In many ways "The Death of Superman" was the beginning of the death of American comics.
Hot take.
Having been involved in the comics industry in the retail segment since 1989 through 2006, and as a fan thereafter, I agree Superman 75 (Nov 1992) was an inflection point. But it was only one strong data point in a sea of many from that era.
I might say X-Force #1 (Aug 1991) wrapped in plastic with hundreds of thousands of copies sold and unread was an early sign of comics sold as collectibles, instead of reading material.
Or maybe the publication of Wizard (July 1991) that focused on the value of comics as objects of investment in addition to (or instead of) creative efforts to be enjoyed.
Or maybe the creation of Valiant Comics (Magnus Robot Fighter #1 May 1991) in direct competition to DC and Marvel paved the way for Image Comics and their way of doing business.
Definitely the early 90's marked the beginning of superhero comic books as collectibles qua collectible. It also marked beginning of the end of a work for hire publication system that benefited from the work of creatives but kept the profits for the corporations themselves.
So yeah, maybe it was "the beginning of the death of [one conception of] American comics". Probably a good thing overall.
If you look at graphic novels and non work for hire publishers (Dark Horse, Image, Fantagraphics, Drawn and Quarterly, etc), and the quality and breadth of genre of that work, I'd say comics are doing better than ever.
If you admire as I do the creativity still coming out in graphic fiction form, I'd say comics are doing better than ever.
If you see all the properties in the 2000's and beyond that are being made from comics, with the financial benefits accruing to those creators, I'd say comics are doing better than ever.