Advanced fares serve a purpose. It is disposing of excess capacity. It is like when the supermarket sells off excess croissants for 19p. You have to do it in a way that most people will still purchase full price croissants most of the time.
I can see what you are saying, but it doesn't /quite/ work like that - If I'm in the house of toothy comestibles to buy toothpaste and spot some of your 19p croissants I might think to myself 'that's a jolly good deal, I'll have a few of those for the weekend'. Excess croissant capacity solved, and a happy customer. (And this is what Mrs M did when she went to the Co-op to buy veg' - she saw some 'Indulgence' cakes being sold off cheap - they were very nice).
But, if I want croissants, then I want croissants at whatever price, and I have no other option other than to pay the price (or not buy them), if they are cheap, then I'm quids in (or pence). I can't pre-book my purchase of croissants at the reduced price.
Back to rail fares - If I want to travel to Oldham* then I want to travel to Oldham, and I've got no choice other than to pay the price (or not travel), if I can see a cheap advance fare then I'm quids in (most certainly a lot of quids).
But, I'm not likely to see a cheap fare to Oldham and think, that sounds like a good deal, I think I'll go to Oldham to see what bands are playing at the Palais**
The railway companies aren't selling off excess capacity - I think what they are doing is demand management, or load management.
The Train Operating Companies don't want everyone travelling at the same time, they want to use their very expensively leased rolling stock as efficiently as possible, so they will sell the first n tickets (note I didn't say seats, oh, no) to Oldham at the price they think they can get away with, knowing that those who are price conscious, and need to travel will but them at that price. As the journey time approaches and these tickets are sold out then they will sell the tickets at a higher price to those who either have no alternative to travel today, or don't care about the price.
It is all far too complicated and does the average traveller no favours at all. Then when you throw the nonsense of rail passes for everyone who hasn't got a rail pass it just gets stupid. (I've got a Disabled Rail Pass because I've got a hearing aid - go figure)
*Why Oldham? - don't know - I'm sure it's fine place.
** No idea if such a venue exists, it should do.