If DoH didn’t exist, a device manufacturer could trivially reinvent something equivalent. DoH isn’t the problem.

They could also do that with out DoH, they dont because it is not "trivial" and prone to all kinds of downsides.

DoH is the problem here, as it hides things from network operators making it harder to block ads, spam, and other items at the network level under the guise of privacy, when in reality DoH's actual goal is to further centralize the internet into approved gate keepers like CloudFlare and Google.