ROTFL, I think we have just found our first troll. (no offense meant, it is just fun to get some arguments going).
Please, in Glorantha we are the Uz.
I think Mongoose need the old customers first.
Ok, me bad. The Uz
Sorry for the long post...
Yes, they need the old customers first. They need to establish a strong foundation, before they can expand the product line with more settings. That is why they release Glorantha first, because it is the setting that is associated with the word "RuneQuest".
However, to make the game last, they also need to publish more settings, to attract other group of players, as well as other setting designers, who might want to take advantage of an RQ OGL insted of what WoTC has to offer (I personally find working with d20 be a tiresome thing).
So I do not argue with you there. But consider that there is nothing that prevents them from putting a general fantasy setting (it does not even need a name, it only needs rules and creature statistics, and a very short description) in the books that mainly contains rules-mechanics (I do not foresee books heavy on rules on the horizon, but I do foresee non-Glorantha specific books).
It would be awesome to have a rule-set that is very much a evolution of the basic role-play system, but with an OGL to allow setting and game designers to use these rules, like WoTC did with d20. The d20-wave is dying off, loosing its momentum, and players are looking for new games, for new systems. But not all are ready to learn too many systems, and quite a few setting designers are not too keen on developing their own rule-system to go with their setting.
Our hobby is not a large one, and only very large companies can survive, as the battle for the consumers money and time is fierce. As happened with the d20/OGC license, the RQ OGL can give small companies a break, the chance to produce their settings, their games, without having to spend a lot of money or time developing a rules-set and try to market it, they can use RQ and provide the players with something that is not d20 at the same time.
The future of our hobby lies with small companies, often only a one-man, part time, publisher. And OGLs are the perfect way to build a brand and give these small publishers something to work with. If you do not believe me about small publishers being the future, take a look on how much the independent scene of RPGs have evolved over the last years (home-brew rpgs that gets published on the net).