We already see signs that it might head this way. Numerous IoT junk (Facebook portal and Google Home Minis come to mind), will ignore DHCP-provided DNS servers and use their own (usually 1.1.1.1/8.8.8.8 respectively) if they don't get successful connections to their mothership. My devices are old enough that they don't try DoH, but I'd be shocked if most of these haven't moved to that by now.