Yes, it is fair.
I will never use my phone without a case.
Especially, because phones became too expensive these days.
€350 is my ideal price.
And I struggle to find a good phone at this price.
OnePlus let me down, and only Xiaomi (and its sub-brands Pocophone and Redmi) make balanced cheap phones these days.
Realme and Meizu are bad examples, in both quality and software experience.
Only Huawei (and Honor) can come close, but their Kirin SoC do not convince me, either due to future support, but also due to the availability of Quick Charge chargers, which only work on Snapdragon SoCs.
Summing up, only Chinese OEMs are offering balanced phones: good specifications for an adequate price.
And a case – and tempered glass – are mandatory to protect my expensive phone.
My Xiaomi Mi6 (from 2017) is still good: up-to-date on Android (and its security updates, fast in every task, the battery last a day (even when I use it hard), and it looks very nice.
Thanks to a case and tempered glass it survived to many drops it took during the 2 years I have owned it. And, only in 2020 will I trade it; mainly because it will be the last year of updates, but also because a 30% improvement will be felt.
Improving 10% each year, for €300-600 for each phone is not a good deal.