Unfortunately at this point in time it would cause far too much interference with game performance.
The way that I understand it (and remember that I'm so not technical it isn't even funny, so some of this may be wonky and one of the guys can pop in and correct if necessary) the server pings the db every time you zone. When this happens it pulls ALL of your character information. All your quests, all your achievements, any and all info that is stored for your character.
So for highly populated servers, highly populated zones, etc it causes considerable strain on the db to have even more information drawn from the db. It doesn't depend on the client side ability to handle the load, it's the information flowing to and from our stuff.
So for now, upping the limit in the quest journal would do more harm than good.
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