Well, that would mean the score gate thresholds would have to be much less arbitrary than they are now, for one. We took a shot in the dark with score gates, as you'll recall, as we had too little data on what scores were reasonable to attain. There is also a "score gates off" mode.
I'm not saying something along these lines couldn't possibly be done, but I'm not sure how wise it would be at this specific point in time. I'm also not sure how constraining that would wind up being for potential future victory conditions: depending on the chosen score gates, you could hit those super quickly and then have no chance to really play the alternate game modes...
And frankly, just choosing the lowest score gate and then really hard other settings would be just the king of cheese. You could "win" very fast on very hard settings. I think there would be major backlash there.
In short: the game was never designed to end early. You start messing with that, and there are consequences. Boredom after you reach the score gate is unfortunate, but I'd rather solve that problem rather than just chopping off the rest of the game at that point.