Well RTS might be a marketing term but when you use that term expect to hear about games around the time the term was created. People hear the term and automatically think of games like dune 2 and what came after.
If you would have said FPS people would have said doom, or wolf3d even though there were other dungeon games with the 3d ish look where you explored and killed monsters.
Some games had such a good combination of design factors that they DEFINED the genre that gets named after them. Sure there are games that are kind of like it.. but none had the same effect as the games mentioned above.