You didn't say it's small, you said it's "trivial". This isn't about people taking pretty pictures, this is about people trying to do actual work. It doesn't matter if it's 1% or 100% if it still ruins what they're working on. So yes, you're being arrogant by saying that just because it's a "trivial" amount that's ruined, when you don't understand what they're doing.

2% is trivial. They lost 2% of information (as someone pointed out this might be closer to 20%). This means the cost to the astronomer was 2% (or 20%). Calling this "ruining" is an exaggeration. Of course it would ruin a pretty picture, but "This isn't about people taking pretty pictures, this is about people trying to do actual work." where you can choose the part of information you take into account.

Except if the thing you're trying to measure is in that 20%. Astronomy is time limited, you may never get the opportunity again.

It is a small part of an image, but that does not mean it's not a significant disturbance.

Think for instance about CO2 levels. Is it such a big deal if they reach 1% of the atmosphere? It's only a small part after all.