My thinking was more along the lines of old vulnerabilities in VLC (specifically codecs/implementation) exploiting a set of the most commonly sold TVs, and spreading via torrents. If your malware group can target 6 models of the best selling 5 year old TVs and spread via torrents and then infecting video files, which spread over Windows networks and keep infecting video files, it could be a good few million device strong botnet.

Seems more like something an APT actor would focus on because the effort:reward ratio isn't there for most groups, and it would take a lot more effort than the MicroTik botnet or other compromised router nets.

I'm hesitant to run any outdated network-connected devices on my (read: the one my personal devices use) network. The only older model device we have running is a brother printer but it still receives firmware updates, and it's segmented so printing is never done directly from anyone's device, it's hooked up to an old laptop running a simple custom web server that accepts files and puts them in the printer queue, and tunneling and DNS are configured on the router, if someone needs to print, they go to [thenameoftheprinter].com in their browser and upload the file(s) and it prints. Devices without access to the guest network can print with Bluetooth, it just requires opening the laptop and pairing and manually printing.

But that was born out of issues of compatibility with the printer running on the guest/kids network, and not wanting to plug it directly into the router or use the Brother apps more than "This printer is older, must not have direct network access."