I don't think they will plummet, actually I would more so see them going up even further lol.
Collecting has been weird lately, be it due to youngins getting a port of an old game and thinking retro collecting is cool after to get the original or that it is simply becoming quite the fad. While there were a ton of earthbounds produced it was not even in the same spectrum as FF7 which had 2 black label releases and greatest hits, if anything the game stayed around retail price long after but consider those greatest hits sold for 20$ brand new lol.
As someone else mentioned the SNES market is tricky, a good example look at syndicate , 20$ game till the current gen version comes out now its been doubled and some cases tripled. Chrono Trigger only went up when the DS version is dirt cheap, mass produced and arguably superior.