Potential complements

Inserting 得 or 不 into a result/direction complement to say whether the action CAN reach its result: 吃得完/吃不完, 听得懂/听不懂, 看得见/看不见, 买得起/买不起, 来得及/来不及. Why the negative 不 form dominates speech, how it differs from 能/可以 (inherent achievability vs permission), and why it is not the manner 得 of 跑得快.