That's exactly the question that is in the back of my head the past few days.
Computer / iMac? Uh, well, having used Linux on the desktop (various distros) for years already, this won't be as much of a problem.
Something that gives me pause is how well Mint runs on some brick-ish HP laptop from around 2012, just replaced the HDD with SSD. It just feels way snappier compared to my fully decked out late 2017 iMac 5k with 64GB RAM, 4,2Ghz.
I'll be missing the deep integration between iPhone, Watch - but other than that not much probably.
But the iPhone itself? That is going to be tough.
Been an iPhone user since the 3G, iOS 2.0 (2.01 shortly afterwards) and always liked the devices, the clarity of the OS (though sometimes it fell short regarding having all the bells and whistles, early on i've solved that via Jailbreaking).
So, no - i don't immediately have an answer. Ideally, there would be something akin to the polish of a modern Linux OS on a decently powered phone, in a nice hardware package.
I dread running Android :(