One of the current achievements in the Eternium Man, which requires the PC to get to level 50 entirely in the SMC. Setting aside for a moment whether or not this challenge is possible at all without scripts, there's a problem that it is essentially impossible to complete this challenge conventionally (ie. just be straight levelling) because the rate of monster generation decays over time. After spending ~20000 turns in the SMC, if you manage to live that long, you'll find that you'll only see maybe 1 monster every couple hundred turns or more (enough that you can basically search the entire level and maybe find 1 monster), whereas at onset, new monsters seem to spawn essentially continuously. Because of this, getting passed about level 20 or so will take forever and you'll end up either being corrupted to death or starved to death.

So, if Eternium Man is to remain one of the achievements, I think it makes sense that the spawn rate in the SMC remain the same regardless of how long you spend on the level.