The thing is that phones can be lost, robbed, broken more frequently than a Desktop. It is more a safety thing than a security thing I think.
Exactly this. No matter how safe you coins on your Smartphone are if you just lose your smartphone. Same goes for laptops btw. That's why I prefer a trusted online wallet.
OTG cable with trezor while using the smartphone to send the transactions seems safe. Keep the trezor hooked onto your keys possibly.