Saying that models will get attracted to bullshit local maximum is similar fallacy to saying that wikipedia will be full of rubbish when it was created. Forces are set up in a way that creates improvements that accumulate, humans don't represent any ceiling and unlike humans models have near zero replication cost, especially time wise.
Like there is only so much you can do with a single punch card.
For example information communication rate that humans can perform (read or write) compared to what computers can do.
Same with information storage, retrieval, precision, computation rate etc.
If people are worried about a runaway effect, why would you think you can dismiss their concerns by constructing a very specific scaling function that will not result in a runaway effect?
Were it to improve by 0.5, then 0.333, then 0.25, then 0.2 and so on would be an entirely different matter!