This would have been true when RuneScape was released, but because of the explosion in social networking and social media, exposure on websites like Twitter, Facebook etc. is capital in its own right. The more players you have, the more social capital you have as a company to use. Unless you're purposefully attempting to give a product some kind of niche market, or some kind of social prestige that only few can enjoy (comparatively expensive "limited edition" offers, for example) it's no longer the case that five people paying £1 each = one person paying £5.
There's no difference between 2 players spending £5 each on membership or 1 player spending £5 on membership and another £5 in the store. Player numbers aren't the be-all and end-all. Even if they are going down.
The difference is four more people talking about said product to their friends. That's why declining player numbers are a worry, and microtransactions doesn't necessarily compensate for that.
NB: By saying that, I'm not arguing RuneScape is necessarily dying. That's far too simplistic an argument. The genre on a more general basis, however, has been in a fairly consistent decline for a good few years now.