People always propose the idea of combing/cramping binds per slot. It was rejected multiple of times for various reasons.
Main and one being that it would ruin diversity and stragedic thinking when carefully choosing the abilities. We can expect players binding slots with normally, but will often than rarely leaving the 9th (as you propose it should be the box for more 9 binds) for all rest left out slots, making everyone omnipotent in that bending art or chi bloking. We all want to bent and use every move as possible, but this would make heck easy and you included in the vote pole "No , It's too* overpowered". It would not be that accurate from the show, because we never seen a bender doing all techniques. In
non-game perspective sense, binds = knowledge/skill style. People aren't perfect, and it would be weird If they could bend every single technique in the existence by the game given choices.
Imagine how it would be unrealistic, If an earthbender could catapult, earthshard, earthblast, earthsmash, earthtunnel, collapse, tremorsense, earthsmash, earthwave, mudsurge, lavaflow, metalshred, lavadisk, earthline and ect. at ONCE per battle. I know that is possible with a workaround by quickly changing binds through a command, but with your implied method its gonna be a lot EASIER as you won't need to change it while using have a good reaction and (typing) timing skills.
The Other being probably as you mention: overcomplicate things. It is definitely a lot harder for (concept or not) devs to think of ways each can collaborate and presumebly coding it.
