But if you trigger a combat in certain areas then that should trigger a flag somewhere and if you long rest without having cleared the area then that should trigger a reinforcement event and then the area ends up being a lot harder.
Or rather you would get an injection of extra XP in a game with a set amount of XP, making long rests in hostile areas a desirable strategy for a quicker progression through levels
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That would be an unfortunate potential side effect, yes. Can't be halped, I don't think.
But you would not really get to high levels all that much faster, not by simply murdering goblins, just like murdering city guards wasn't really a great strategy for reaching higher levels faster in the original games. There's enough experience to go around anyway and there are vastly more interesting ways to earn it than by spending a couple of hours fighting a small legion of goblins. Unless one happens to be looking for that challenge, of course.
Let's be honest, we've probably all done the infinite giant slaughter in Saradush, at least once, but how many of us would regularly leave the computer on overnight to max out experience that way? And that was an infinitely respawning giant worth thousands of experience. All in all, I think certain areas getting reinforcements if you take a long rest before finishing them is a perfectly sensible compromise that provides some incentive for not resting after every fight but also doesn't really force any kind of grief or meaningless busy work onto players.
