By the same logic HTTPS is bypassing your network filtering of every service which happens to have an alternative available over HTTPS. So do you block HTTPS?
I would also love to MitM myself in some cases, mostly because it’d be interesting to see what’s going on.
So before applying that mentality, it would be wise to consider what your experience would be like if all your neighbours, friends, colleagues etc also did that on their networks.
For this reason I wouldn't recommend buying a device like the Chromecast, in which the user can't configure the network settings. Instead maybe consider something like the Amazon Fire Stick which is not as user-hostile.
The usual approach to setting up a firewall is a default of "block everything" and then selectively allow only what is needed.
Most people cheat and only do this on inbound connections, allowing everything on the egress side, because it's easier. But if you want to block your IoT devices from making outbound https connections, you easily can.
There's nothing really new going on here. It's always been possible to tunnel one protocol over another, or use nonstandard ports, and use encryption on the traffic to hide what you're doing.
The point of the original story is "your" meaning a device you own, is ignoring your network controls
it is highly unlikely that the TV manufacturer is going to allow me to install my own custom root certs to inspect their traffic to HTTPS, so yes DoH and other things are a threat to network security, because if the TV become compromised I have limited administrative controls to prevent it other the blocking it completely which is a poor response to the problem
DoH is a solution in search of a problem that can be solves in better more user friendly ways