It may be that players don't want to speak up for fear that potential future coaches/managers may be wary of employing them. Rightly or wrongly, it's easy to get a reputation as a troublemaker.
I agree but there is a difference between actively speaking up and seeking to work with that person again.
Whilst we don’t know for sure, I’d guess that Marchant had multiple options. If Gustard was so awful to work for, Marchant more than likely could’ve gone elsewhere without having to publicly say why.