I really can't make it any more clearer to you than that. But over time as I have explained you can't lose as statistically 70% of favourites lose the races that they run in.
I suspect that you have seen this betting system but never tried it yourself since you don't acknowledge that laying a horse involves a variable cost.
A £10 lay on a 1/1 (Evs) horse requires £10 to cover the bet. A 3/1 requires £30. An 8/1 requires £80 etc. Laying a horse for £10 does not mean that it only costs you £10.
Lets work through some examples. In each one there will be 10 races, we lose the first 3 (the horse wins) but win (the horse loses) on the following 7.
Card 1 - all horses 1/1. Our races bring the following profit/losses:
-10,-10,-10,+10,+10,+10,+10,+10,+10,+10 = +40
Card 2 - our first three are 3/1 shots, the rest are 1/1
-30,-30,-30,+10,+10,+10,+10,+10,+10,+10 = -20
Card 3 - our first three are 8/1 shots, the rest are 1/1
-80,-80,-80,+10,+10,+10,+10,+10,+10,+10 = -170
So in our three card programme we have a loss of -150 and as everyone can see, price is relevant.
The reason I have given such an extreme example is to highlight variance. The more races a person tries this on then the more smoothed out variance will be, but it's entirely possible for a couple of long odds winners to happen in a row and your balance is turned negative. I wanted to highlight this so that people didn't decide to just do this one Saturday afternoon and expect profits when very real losses could occur because of short term negative variance.
People also need to consider that the 70%/30% figure is only a historical 'fact', as in it tells us what has happened in the past. It may be indicative of a trend but this does not mean that it will stay at this figure in the future.
Even if 70%/30% was always going to be true then a real downfall with the 'lay the favourite' guaranteed profit myth is the fact that the price you are laying at is not simply a reflection of the prospects of a horse winning/losing, but also the amount of money in the market and on the selection. As such, the odds are skewed one way or the other and this does affect the calculation.
I'm not trying to have a go at you personally, mate, but I don't want fellow punters to think that a golden goose has appeared for them to then go and lose money.