I would hate that change. I usually hold my phone in my left hand, and can reach the back button with my thumb. Using the swipe gesture at the bottom of the screen to switch apps would require switching to a two-handed grip for that one common action.

It's not ideal; I'd like to see more attention given to multitasking in general, the re-arranging deck of cards system has always been counterintuitive and clumbsy in my opinion.

So is it mainly for lefties? I hold my phone with my right hand and that button is inaccessible. Based on the downvotes, I feel like I must be missing something.

iOS is kinda a mess when it comes to what's easy for people who hold their phones in their left versus right hands. That back button is definitely for left-handers, but then the control center is clearly better for right-handers. The slide-from-screen-edge-to-go-back gesture is only usable in the left hand, and the slide-from-screen-edge-to-go-forward gesture is only usable with the right hand. My conclusion is that iOS is only really designed to be used with both hands gripping the edges of the phone, which meshes with their ever-increasing screen sizes. The new Pro Max is 6.9"!! That's basically a tablet.

you wouldn't hate it since a proper implementation would make it optional

There is no way Apple would make a change like that and have it be optional.

It's worse than useless - I sometimes mistap it and switch the apps