Wouldn't a device that travels like a snail along a cable or something, while less glamorous than a self propelled (and navigating) device, deliver a reading for a whole line of plants every day without all of the eroding parts per plant?
Having machines moving around taking readings “each day” is insufficient, as during hot conditions the soil/plant moisture changes too rapidly. You need to take readings at least once every hour or even every 30 or 15 minutes in order to be able to start irrigating immediately when the moisture level starts dropping rapidly.
I can imagine that a lot of small devices becomes economically viable.. I find that unfortunate since I think it will be a lot of permanent ewaste lost in fields relative to very temporary water optimization benefits.