With the current model, it's not too hard to reduce evolution to 0 if you idle long enough.
I play with space exploration, and it's easy to grind the factory to a halt while I setup a factory on a different planet to progress my space station, which can result with evolution tanking.

Some soft of a soft cap, perhaps half the max evolution value you had, which would exponentially reduce reduction (and increase growth) of evolution the closer you get to it would improve it, I think.

Interesting idea. But for evolution to keep going down you need to continously output small amounts of pollution (too fast and you damage the environment which increases evolution), and the change in evolution factor gets slower in both directions when as it gets closer to 100% so this will also make this exceedingly slow. Maybe if you have a a factory spread out over vast amounts of space the amount of pollution that can be absorbed in parallel by a huge amount of trees may actually be anough to counter the time factor?

So how big is your factory (in tiles) and what is your evolution time factor?