in a letter didn't change the issue, other than potentially broadcasting it to a wider audience.
Bronco's departure is a complicated issue that is not black or white in either side. He wasn't forced out. But he did feel some pressure. He chose to leave, but not everyone was disappointed in his departure. But DS's letter played very little--if any--role in Bronco's decision.
If one of the reasons for Bronco's departure was a consensus of former players, then Bronco has primarily himself to blame. There were many, many reports over the years that former players did not feel welcome by Bronco and that he failed to maintain relationships. I believe that TH is not so reactionary to fire a winning coach over the opinions of some former players, although I'm sure he was diplomatic enough to listen to their complaints.