Or we can assume that the state of the world before humans was a state that we have deviated from though human agency, and it is reasonable to remedy some of that irreversible trajectory would be a good cause that some other intelligent species could undertake after we're gone.
The first known extinction event was caused by cyanobacteria that released vast amounts oxygen (highly toxic chemical) into the atmosphere. It killed most of the anaerobic life, clearly "deviating from the previous state of the world". The time has come to "remedy some of that irreversible trajectory".
The whole notion that the "current trajectory" is somehow objectively bad is ridiculous. It may be bad for humans though.
I think you can define degradation. I can make objective claims that less biodiversity is worse than more biodiversity and use it to measure degradation without humans around. I can measure this degradation in planets without humans or model out the degradation in a future where humans are extinct, and call certain situations objectively worse than others under that yardstick.
Maybe I misunderstood because it seems you're claiming if there's no humans around it's impossible to model any phenomena, but we possess abstract thought to make up those situations and define things like degradation in the absence of humans.
> I can make objective claims that less biodiversity is worse than more biodiversity and use it to measure degradation without humans around.
While the amount of biodiversity can be measured objectively, the notion of it being "good" is completely made up. You need humans to judge what's "good" and which means that for any "good" kind of future humans are essential and are more important than say biodiversity.
If biodiversity is good (and I say it is, but it’s a human judgement), then human civilization is also good, and for the same reason: it represents life, complexity, activity, and progress.