Stats are made to increase at the rate they do because that is what happens to optimise the gameplay enjoyment. Focusing too much on realism by using shallow stat growth, and you could end up with something boring.
But the gameplay doesn't necessarily need to be optimized by stat increase. I don't see why gameplay has to be boring just because stats don't advance.
If the stat curve is too shallow, then players will feel a poor sense of progress.
That's only if nothing else advances. That's not going to be the case. Skills will increase with use and they should notice the missions they're assigned being "better".
What they struggled to kill at level 20, they'll still be struggling to kill at level 25. Leveling up loses it's significance (and thrill factor) if you can't feel any difference between one level and the next. WoW is very balanced in this regard ~ you can just about feel the difference between level n and level n+1. Players can feel they have advanced.
I played WoW a bunch of times and never noticed this, at least at the first twenty levels.
For completeness, if the stat curve is too steep, the game can begin to feel linear. "It's impossible to survive this task at level 10, but it's doable and enjoyable at level 11, but if you leave it until level 12 it's not worth doing because it's so simple that it's boring."
And that's why levels aren't going to be part of this. It's an artificial indicator of advancement through the game that's not necessary as long as the player feels he's accomplishing something another way. Leveling isn't the goal, completing missions is. The advancement will be bigger, more complex missions, just like most MMOs.