That's domain speculation: it may pay off, it may not. If people are relatively confident that they know what may become popular in a year or three, then this kind of thing can pay off. You do have to know what trends to watch, though - and then hope that someone else didn't already think the same thing that you did, because then they would have bought the domains already.
OTOH, if just a handful of the domains you purchase becomes wildly popular and someone does decide to buy them from you for a few hundred or thousand each, that can cover the original investment and make you some profit.