Standardization provides endorsement. If DRM requires a non-standard plugin, and plugins themselves become increasingly verboten, at least some folks who might have used DRM may decide that they want to reach a wider audience instead.
Consider what happened when Apple refused to allow Flash on their platform; that certainly accelerated the decline of Flash, or at least forced the development of Apple-specific approaches for that platform.
So what would happen if Chrome and Firefox had stood together in solidarity and said "no"? I don't think the answer would have been a mass exodus to IE; for that matter, what if IE had gone along with it? (Edge already bans plugins.)
Consider what happened when Apple refused to allow Flash on their platform; that certainly accelerated the decline of Flash, or at least forced the development of Apple-specific approaches for that platform.
So what would happen if Chrome and Firefox had stood together in solidarity and said "no"? I don't think the answer would have been a mass exodus to IE; for that matter, what if IE had gone along with it? (Edge already bans plugins.)