There is always a market for a quality horse, no matter what breed. As for the average or poor quality horse especially, they have become a dime a dozen.
There are many "dispersal" sales and auctions every year. Yet, those farms are still on the radar a few years down the line. It is used in many cases to bring buyers out thinking they are getting a coveted broodmare or stallion at a good price. If one goes onto the Stud Book On Line, one would find out that these coveted broodmares where never owned by the seller, till that year . They where mares that where bought for a song at some sale somewhere and bred to their stallion and then re-sold.
As for the stallions, often they are no longer being used by the seller or are no longer producing (perhaps they are shooting blanks).
One should be very careful when buying from a dispersal sale.
On the other hand, there are farms that are going out of business. Either they are getting too old, not making the money they thought they would make in the Miniature World or have decided that they want to move on to something else.
There are many farms who would buy Minis at a rather cheap price, then resell them for more money. The broker farms, who many have now gone under. So, they have a dispersal sale. Heck, they can no longer make the kind of money they used to.
There are the small farms who bought Minis to make a few bucks and have some fun, who did not have quality Minis and have figured out that selling a Mini for a few hundred dollars is not even going to cover the cost of getting that Mini to birth, let alone raise it for a while.
But there will always be farms out there who are striving to produce a quality Mini and are in it for the long haul. They may be cutting back in what they are producing, but are still producing to reach their goal.
Other farms keep producing 20, 30, 40 or 100 foals a year, just because they have been able to sell them in the past and are riding on their name for selling them in the future. (remember, not every foal a top farm produces is quality, many of those farms produce pets too) Then one day, they look out in their pastures and see 200 or more horses and realize they can no longer sell them or take care of them properly.
Unfortunately, this happens in all breeds and there is always an ebb and tide. Those who hang in there, continuing to strive to produce quality animals (though in a lesser amount for now), will come out the other end hopefully successful.