You should be able to configure Template Sensors (Binary Sensors, Switches, Locks, Fans, etc) in the UI like you can currently configure Helpers (input_text, input_boolean, etc).
I realize this is a wee bit more complicated than an input_* because various listeners have to be assigned based on the template’s contents. Also, the listeners have to be modifiable based on changes to the template. Nevertheless, the ability to add a Template Sensor without having to restart would be a big step forward to the day when there’s almost no reason to restart Home Assistant (other than for performing an upgrade).