Well, that happened because in ~2011 the entire media industry announced that they will stop media playback on devices which didn't secure the DRM-keys.
As media consumption was fundamental to the Smartphone ecosystem, Google made secure-boot and widevine mandatory.
Sure, the same could happen here, but I don't know which industry (or other body) would demand that and have sufficient justification for it.
After all, the feature doesn't really change that much for a consumer, the trust-chain is largely unchanged: If I send you a picture and tell you that's my dog, you still have to take my word for it, regardless whether Apple signed the picture or not.
