People rent their networks from one, maybe two area options. The consumer networks want to completely control router hardware these days and these days charge extra rental fees for owned hardware instead of rented hardware. (It's fascinating that they can legally get away with that.) Some of the biggest consumer networks have already proven they are happy to use this hardware control to inject additional ads into customers' networks for a paltry amount of additional revenue.
You are correct that people should have networks that they own and trust at home. You may have missed that they don't and consumers have lost that battle. (You may also be underestimating just how much time people spend on devices "out in public". The mobile device has become the most common device for a lot of users. For some users the only device.)
> every application / Trojan writer
They've always had that power.
Applications have never been forced to use OS/network-configured DNS. DNS is an absurdly simple protocol that doesn't even have encryption by default. OS firewalls might block sockets to DNS ports by default, but there are ways to tunnel over other ports plus tools like UPnP given enough user trust.
DoH is a standardized port tunnel but that doesn't mean that unstandardized ones never existed before. Trojans/viruses have been doing weird things to avoid DNS for decades. DoH doesn't make them that much easier.
DoH isn't great and it is a shame that for privacy and control it's a big ugly trade-off/compromise from ideals. It's useful for some people. There are definitely unanswered questions in terms of which big corporation truly cares about privacy. I've seen my monopolist consumer ISP inject ads against my wishes and do change the DNS on my home (owned) routers (that I pay extra for each month despite owning my own hardware because of owning my own hardware). I don't always know what to think about Cloudflare's massive PR engine of how much they claim to value privacy, but so far I've never seen them inject an ad where one doesn't belong nor have I seen ad revenue make a splash in their quarterly reports. They don't seem to be an ad company. (Yet?)
Trust is hard and we all have different threat models. I don't blame you for distrusting Cloudflare. I have direct evidence for distrusting my current ISP and indirect evidence for distrusting most consumer ISPs I've encountered, despite being paying customers. There's no free lunch and there's no right answer, just a lot of "least wrong" answers. DoH isn't the right answer objectively. But DoH can be a "least wrong" for some users. Just as trying to be the MITM in networks you own is quite wrong from a security standpoint (once you've got one MITM it becomes harder to trust that there isn't a second one) but may be the "least wrong" answer for some users including maybe you.