Timeless.
Art and historical objects circle through different museums all the time, it's not a new concept. "Gosh I plundered something from India or Pakistan, oh my whoever will I give it back to?!" isn't a serious issue
That's not how geopolitics works, particularly in that section of the world.
https://en.wikipedia.org/wiki/India%E2%80%93Pakistan_relatio...
If the British had taken possession of an object from an Indian tribe in California in 1800, who would be entitled to it? The U.S. government? The government of Mexico? The State of California? The Tribes' descendants?
Possession is 9/10 of the law.
By the way, there are many examples of colonizing European countries refusing to return things they took during colonization where that "we don't know who we should give it to" argument is much weaker or non-existent.
For example, Egypt has many items that are on display in European (mostly British and French) museums that it has tried to regain where ownership is not that complicated because the item was taken from Egypt by a European without complicated chains that might affect who owns what.
Maybe the most famous is the Rosetta Stone, which was found and taken by French troops from Egypt, then given to the British via some treaty, and the British refuse to give it back despite many calls from Egypt. You could say something like "oh well it's unclear whether the British should give it back to the French or the Egyptians"... but I hope you can see how that would be a stretch.
From an Egyptian viewpoint - a good example might be as follows:
What if an artifact was found in the Gaza Strip in 1955 and taken to the UK, who should the UK return it to? The Gaza Strip government, the West Bank government, the Israeli government, or the Egyptian government?
Edit: cannot reply below -
1. I'm pointing out a specific requirement in IR
2. The Gaza example above is an analogy for the Kohinoor example. I honestly don't care about the Egyptian case because I have no connection with that part of the world and I assume there are different considerations.
Also the Gaza strip example is just unnecessary hypothetical scenario. The British and french stole a lot of things from all over Egypt. There is no need to imagine hypothetical scenarios.
“We stole it earlier” is not a more legitimate claim.
And in any case, these countries are perfectly capable of sorting these disputes out amongst themselves, the same way they had to sort out much else after independence, like who inherits the armed forces and the treasury.