From what I have heard, JM made a whole lot of enemies, he is literally black listed with the Hollywood crowd. I heard that they even destroyed all the B5 props costumes and sets even though he wanted to buy and save them.
That might be revisionist history on JMS wanting to buy everything. He wrote nearly every episode after the first season. He was reported working 20 hours per day. When he finished the series, he had used up all the ideas in his story arc.
When the fifth season was over, they wasted no time destroying sets. The series was filmed at a former warehouse in Sun Valley CA which was owned by the production company. I seriously doubt Warner Brothers owned the props or sets. Certainly they didn't have possession of them. It is my opinion they were destroyed to prevent Warner Bros from recreating the series without JMS' control. They also needed room in the studio for the Crusade sets.
The CGI models were in JMS's possession and he stated several years later that the hard drives that the CGI models were on were no longer in existance (with implication again to prevent an easy series return). Blowing up B5 in Sleeping in Light was an intentional attempt to prevent a series resurrection.
Now there might have been bridges burned, but there was a greenlight for preproduction on a theatrical B5 movie, surviving the death of Richard Biggs and then ended with the loss of Andreas Katsulas.