We all have our own concerns with DC's output. But our individual gripes do not necessarily correlate to DC's issues. Their issues are at least two-fold:
1. Their characters are available in other media, which if it drives interest to comics, draws that interest to the classic stories that served as inspiration.
2. Similarly but more importantly, their new stuff competes against the best of their old stuff for relevance.
That can't be overstated. Their classic books and runs serve as better introductions to the characters than their modern output - O'Neil/Adams Batman, Batman Year One, New Teen Titans, All-Star Superman, etc.
A tangential concern is how interwoven and insular comics became over the last 25 years, as a result of playing exclusively to the longtime fan's preferences -sooo many characters, individual stories being driven by a "universe" arc, etc.
I can't quantify that issue, but it's there.