And sadly, so many of the main cast are no longer with us—far more than one would expect from their ages.
I know that some of them, indeed, died due to substance abuse issues; I don't know the circumstances of all of them. They will all be greatly missed.
The curse of Babylon 5 https://www.imdb.com/list/ls056207990/
In the lingo of the show, they have "gone beyond the rim" (of our galaxy).
Being a B5 fan, I found it heart-breaking to see Jeff Conaway on the reality show Celebrity Rehab a decade later. (the fact that there even existed a TV show called "Celebrity Rehab" ...)
Too many others of the main cast had died relatively young, of "natural causes" though. Richard Biggs, Mira Furlan and Stephen Furst stand out.