Which is a different thing to your LFs actually increasing by adding another flight.
It's pretty basic maths. Say it's 400 pax, with 80 seat planes. If you have 6 flights, and 2 competitors have 1 each, then sure, you'll probably get a bit more than 75%, a bit more than 50 per flight, a bit more than the 62.5% LFs you'd get if it was strictly in proportion. Let's even say it's 10% extra, 55 pax per flight, and only 35 each for the 2 competitors.
However, what you're saying is that if you remove that 6th flight, LFs will actually drop. That your 5 flights vs 2 singles will give you less than 55 per flight. 5 x 55 is 275, that leaves 62.5 each for the two singles, higher than you're getting. They should get less than you do, with your 5 flights vs their one. Therefore, you will get more than 55 pax/flight yourself.
Adding an extra daily flight to an already oversupplied route will NOT increase your pax/flight.