Modern art is not art in the sense it's not a highly crafted work. It's minimum effort.
The time frame was given to acknowledge the fact that we think this is "art". In the future we will know this is junk and not art.
I really dislike most pop art, but at least it's art. A big yellow square canvas in no way should constitute artwork.
There are literally factories in China where painters paint replicas of classic art works at scale. They paint hundreds, even thousands of such pieces every year.
They're undoubtedly very skilled painters. But would you call their work "art"?
How much time and effort something takes has no bearing on whether or not it is art. Something that takes 5 years to make isn't automatically better than something done in 5 minutes.
Painters like Picasso and Matisse were also classically trained and had all the skills to paint like the painters of old. They just chose not to.
Additionally, the fact that you're judging the relative worth of a piece of art based on it lasting for 1000 years is pretty sad, in my opinion. What is wrong with art being created for a specific time and place? Life is ephemeral, and there is beauty in that.