You have 2 things wrong right now.
1. RS is a different type of game that appeals more to the WoW Classic player than a modern player. Not to mention Classic RS.
2. RS has more players than Classic RS. It's consistently been this way forever. Modern RS always maintains roughly 30% more players than Classic RS. Jagex doesn't even try to hide this fact either. It's advertised right on their website. 100,000 people online. Classic has about 80,000 people online. Yes, these number will fluctuate, but only sometimes does Classic actually surpass Modern and it's not for long when it does. Consider that most of the OSRS player base is all that's left that even plays the RS at all anymore and yet they're still surpassed by modern stuff.
Overall, what will probably happen is the same thing that happened to RS but with higher player counts.
1. Classic launches. A massive onslaught of people play Classic. Old and new players all hop on to experience it.
2. Classic is a few months old. Nearly all players that didn't play Classic in the first place are back to modern WoW/other games.
3. Classic is half a year old. A lot of the players who wanted Classic realize that it is, in fact, an old game, and stop playing.
4. Classic is a year old and most players realize they've become bored.
Classic is left with a small player base. This is how it happens to RS. This is how it happens to all nostalgic things. Classic is an attempt to save WoW in what Blizzard possibly sees as being close to end-of-life. It will bring many players back, but it won't save the game and the majority of players will be temporary. Blizzard will make a massive dollar day 1, but will see an incredibly massive dropoff by just a couple short quarters later. This is what nostalgia does to people. It's hilarious to me that people think Classic WoW is going to be some magical exception to a rule that has always ruled over Nostalgia. It's not that I don't want them to be an exception, but I'm not going to lie to myself about the situation. I highly suspect Blizzard is 100% aware of everything in this post already too. They will probably use the immediate, but not steady, funds that Classic will bring to fund a new project.