One has to look carefully at the timing of these things to see what is really happening. Here is a brief history as I understand it, as to how the market has recently evolved in ancient coins.
Prior 1975 probably about 70% of the coins on the market were recycle of old collections, and 30% chance finds from construction, river erosion, field walking, etc. Exact percentages are hard to pin down, but that is somewhere in the ball park.
The introduction of effective metal detectors about 1975 very quickly changed the percentages to about 70% fresh finds, and 30% combined recycle of old collections and chance finds. The number of coins from collections and chance finds did not diminish, it was simply an increased total supply due to metal detector finds. This happened mostly in Britain and western Europe so large hoards of Roman and western Greek coins, including coins from Italy and some from Greece, were what was available and prices for those fell. We did not see a lot Roman colonial or Eastern Greek coins.
About 1979 the US Government changed a law allowing rare coins to included in those tax deductible retirement savings portfolio's. Hucksters promoted it and many people who knew nothing about coins bought anything the promoters offered, creating a huge demand driving prices sky high. Common decent Gordian III antoniniani were $125 with someone even attempting to create a futures market for them. Coins of all types, not just ancient, were driven to very high prices. A lot of real collectors sat on the side lines during this period. Late in 1980 or early in 1981 the US repealed that law disallowing coins in those investment plans, creating a market crash that hit bottom between 1982 to 1983. Those $125 Gordian III ant's fell to about $12.50, and much of the rest of the market did similar things. When the market hit that bottom about 1983, collectors jumped in again and the market began to recover.
Around 1985 Britain and western Europe had been very heavily metal detected and finds started dropping. With the reduced availability prices began to rise. The percentage of coins on the market from old collections started to become a bigger factor, although metal detector hoards were still appearing, just in smaller numbers. The market hit a peak about 1990.
In 1991 the iron curtain fell and everything changed. During 1991 and 1992 there was a flood of coins out of eastern Europe, which were the thousands of hoards found and held by families not willing to take the risk involved in getting them out during the previous 60 years of communist rule. These were mostly the types of coins found in the Balkans, so huge hoards of 3th century AE antoniniani, colonial bronzes, 2nd and 3rd century denarii, new style Athenian tetradrachms, Alexander tetradrachms, etc.
Between 1993 and 1995 metal detectors flooded into those regions and fresh hoards began appearing on the market in large numbers, continuing until about 2001. This was the period of cheap coins because there were just too many coins for the collector market to absorb them all. That was partially alleviated by the appearance of eBay with the wider marketing to an entire new group of collectors it allowed. If you had been at a major coin show in the US or Britain at that time, you would have seen many new hoards offered at every show. By about 2001 most of those easy finds had been found, and the number of hoards diminished rapidly over the next few years. With reduced availability, prices started to rise. By 2010 one could go to a major coin show, and sometimes no new hoards would be offered. Dealers started to notice they were having a great deal of trouble replacing their inventory. Prices were rising.
Since 2010, the supply of new hoards had dramatically dropped as they had simply running out of places to look. Recycle of collections is again becoming the dominate source of coins on the market.
Going forward I feel a few prediction are possible. Metal detecting will continue to diminish as a source of new coins, because all the easy finds have now been made, and there are no new areas that will eventually come open that have not yet been searched. It will probably never drop to zero, but will never again be like it was prior to 2001. Chance finds will also diminish because many of the hoards found by metal detectors between 1975 and 2001 are the hoards that would have been chance finds over the next 200 years had metal detectors never been invented. The primary source of coins on the market will from here on out will be recycle of collections.
Taken together, this means that availability of ancient coins had already dropped significantly from the 2001 highs, and will continue to drop. Prices have already risen to a point it is driving some collectors out of the market, and so with the reduced availability we are also seeing some reduction in demand, although that is more true for the cheaper types of coins (serious wealth collectors are still there). I believe it will take 10 to 15 years for things to really stabilize, and it will take the form of reduced demand and prices for the more common material, but high end coins will go higher. I guess in 10 to 15 years we will know.