There is no moral grounds for stealing cultural artifacts. Even if it means the culture that rightfully possesses it wants to destroy it. That choice is entirely that cultures decision to make. Even if we disagree. It’s one thing to clutch your own pearls but so much worse to do that to someone else’s.
While there can definitely be some legitimate discussion and ambiguity over which culture/country gets to inherit Mayan artifacts, for example, saying that the British, for example, should inherit it is a very weak argument. It’s not like the entirety of an extinct societies people just dropped dead. Some survived and after some time rebuilt new societies. Using Mayan artifacts as an example, Mexico, Guatemala, and Honduras have a better claim to them then the British. It’s not propaganda or useless to say that items of cultural heritage should be returned.
So how about this what about-ism, if you live in the United States, the British took cultural artifacts from your lands too and aren’t giving them back right this moment. Where did you think all those native American artifacts in British museums came from? They didn’t make them and it’s not like North America was spared from British plundering. Might be nice to get that stuff back.