Serious or not, but for example Diablo Immortal is mobile only and it reportedly brings in $1M _daily_.

So some people are seriously playing it with no physical controls. =)

For "serious" mobile gamers there are devices like the Razer Kishi: https://www.razer.com/mobile-controllers/razer-kishi-v2

On this site people tend to forget that there actually are people under 30 whose only computing device is a phone. They use it for everything.