An opinion I've heard a few times, but a flawed one. Pandas aren't like a dying person on life support, they're a dying person on life support who is dying because you're occasionally stabbing them.
The inability of pandas to survive is a direct consequence of humans destroying their habitats. So it's a bit hypocritical for humans to not support their conservation. The same would happen to any other species.
In addition, your premise is factually untrue. Just last year the giant panda was reclassified from "endangered" to "vulnerable", which is the conservation level just below "non-threatened". Their numbers have been increasing, and there have been rebounds from similar numbers in other species such as the bald eagle.
Biodiversity is something we don't have the technology to restore from extinction. Any losses to that are permanent, as well as the effects of that loss. It's not just foolish but suicidal to allow ourselves to say one species should live and another should not when we in fact know so little about the consequences on other species, including humans. The dust from the Sahara feeds the Amazon. You don't want to wake up and find out we fucked something up that we can't fix.
And finally, frankly our efforts so far have been pathetic. The conservation movement worldwide survives on a small drip of funds and does great things with it, but would do even more if we valued it correctly. There's not a zero-sum game being sucked up by the adorable faces of pandas, there's just a failure by human beings to prioritize what we do with our resources.