Someone should make instant-pot like device with integrated magnetic stirrer and accurate temperature control.
They are fundamentally different cooking techniques. Immersion cooking, "sous vide", is cooking food in a sealed bag in a water bath.
"instant pot" are more like smart cockpot/pressure cookers where the food is contained a pressurized (sometimes) vessel in air.
Which is better depends on the type of food and cooking technique you wish to utilize.
I get what I can do with a pressure cooker that I can't do with a circulator.
I do not get what I can do with an unpressurized "smart crock pot" that I can't do with a circulator.
Also, the pressure vessel means you can go to water temps above 100C. Not sure if that's a useful/interesting range for most ingredients, but some creative souls might find a use for it.