The majority of horses I back I don't expect them to win, the price they are paying tells me that its unlikely to win the race, I have in my opinion increased my chances of coming out ahead because I am backing it at overs, but unfortunately I don't have the ability to influence the outcome of the race. I can only bet on what I know long term works.
The last statement from you is basic economic sence, if you continually take unders you will lose long term, taking unders is exactly the same as playing pokies long enough you will lose. The house in race betting has odds stacked in its favour and will eventually win if you keep betting horses at less than your theoretical chance.
Constantly betting on overs and with an edge DOES NOT guarantee you will win long term either, you have increased your likelihood but thats all. Handling a huge run of outs mentally is far more important than any selection strategy ever will be.