Not all collectors are interested in the same coins, some only collect ancient coins, some only modern, some collect gold, some silver. The Roman Empire minted millions of coins. Many are still available today. It is possible to buy large quantities of Roman coins for only a few pounds, sometimes less. Most of these Roman coins are over 1600 years old, but although they are interesting, and very collectable, they will never become highly valuable unless they number of Roman coins collectors starts to exceed the number of Roman coins available to collectors.
Coins are a form of money.
People do not go around throwing money away. Even Turkish Lire, worth about one penny per million, or Yugoslavian Dinars which are inconvertable. This means that old coins just hang around in drawers for decades before resurfacing, often in near mint condition. If you have picked a coin out of circulation which is only worth face value now, it just may eventually acquire a premium value to a collector, but a similar coin in mint condition would always be more desirable to a collector, and hence more valuable. If a coin is rare and valuable now, then it will almost certainly be worth keeping, and will probably tend to increase in value.