Honest answer for a farmer to when the glut of fruit should come onto the market is probably never. This is a situation of supply and demand. If demand is high and supply is low price is at a premium and the businesses make money. This is not a monopoly but rather good market practice by lots of growers, some middlemen and certainly at the end of it retailers. Unless a retailer has a contract with a supply chain from grower to store warehouse they have little control over the supply.
Years ago one of my uncles grew a massive crop of pumpkins but so did every other grower in the region. They went to sell this huge crop but were offered very little, in fact so little it was way below the cost of even the seed used to grow them. What did they do? First they held off harvesting for the short period they could, then they just plowed the majority of the crops in. Went from a glut to near scarcity, at which point it was profitable to harvest. Point is that farmers sometimes are too successful for their own good and when this happens they may hold back product to keep prices inflated. They are somewhat insulated from a public backlash by the supply chain starting with retailers being held responsible for the prices, then the middlemen for their cut...the farmer is largely ignored by the public for the cost.
But you do raise the valid point that when retailers control the supply they can manipulate the market as well. For example the price of milk that was retailer branded and they set a cost that was even below what a dairy could survive on.
I don't think in this case it is the retailer who is manipulating or perhaps the majority manipulator but rather I think it is the farmers who realise their profits are at risk if they flood the market.
Well yes and no. The yes part is they can last on the trees, some up to 8 months and this is a good way of storage as it doesn't incur many on-going costs. But the no is equally valid in that tomatoes, bananas and some other fruits and produce can be stored in ways that allow them to last for, in some cases, years. Controlled temps, controlled atmospheres, chemical sprays & dips are some of these ways. Tomatoes are generally picked green as are bananas and they are then stored in particular temps and even gases such as high nitrogen atmospheres that inhibit the aging/ripening. When required they are gassed with Ethylene (C2H4, also known as ethene) and this starts the ripening process. Wheat at below 10% moisture can be safely kept for years with little or no deterioration in quality, other grains are similar.
Avos do not store well at all in cold before they are ripened and so they are at a disadvantage to some other produce when a glut occurs.