I think iOS needs to be a lot more aggressive with this, in both directions (switching because of poor LTE and because of poor WiFi). If it were better, I don't think I'd never actually need to touch the WiFi toggle.

The problem here is that a lot of people are on limited data plans. So if wifi is poor in one corner of my home, I dont want to accidentally chew up half my monthly data allowance.

Where is this setting? I remember it being on an old beta (7/8) but I thought it was removed?

Edit: Found it. "Wi-Fi Assist" in Settings > Cellular / Mobile Data > Scroll past all the apps > "Wi-Fi Assist"

iOS won't detect when I'm on a good WiFi connection but its ADSL link has decent ping yet is much much slower (10Mbps/0.6Mbps) than what I would get with LTE (40~80Mbps both ways) at the same location. I'm on a 100GB plan so I could care less about data use, and I have FTTH (300Mbps/100Mbps+sub-10ms ping) at home so I want WiFi there but always forget to toggle it back on. This UI solves it for me, although I wished the 3D Touch UI would allow quick access to the third, disabled state.

Unfortunately this isn't a good option. My Pixel XL on Project fi is supposed to do this. It never does it when it should. Never. I have yet to see a phone handle the tradeoff well.