We've been using liquid hydrocarbons capable of thermal runaway (petrol/gasoline) for a century despite the existence of liquid hydrocarbons which are more resistant to ignition (diesel).
LFP chemistry will certainly become increasingly dominant over the next few years, particularly now that key patents have expired. But it doesn't supplant all usages of NMC yet, and isn't likely to for some time.
You're mixing up LFP (LiFePo4 aka Lithium Ferrum(Iron) Phosphate) with NMC (Nickel Manganese Cobalt). NMC are lighter and more firey.
LFP can have similar density since you don't need the same amount of protection in the pack. It's a system level win.
Iron ion and phosphate are the same LiFePO4. Compared to cobalt ones it has lower nominal voltage, and higher recharge cycles, less energy density as well.
> Tesla had to buy the technology from BYD.
I'd be terrified of driving a car with byd battery tech. There are countless videos of their cars just spontaneously combusting while driving on a road, sitting on a parking lot without charging etc...
I'd imagine most Model Ys are LRs. The standard range wasn't even listed normally until like a month ago.